Add example scheduled certificate check

Additional certificate expiration check playbooks:
  - one that that uploads the generated reports to a location in the masters
  - one that uses custom report paths with timestamps

OpenShift Job and ScheduledJob examples that use these playbooks
run from a container to check cluster certificates from within
the cluster itself.

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+# openshift-ansible usage examples
+
+The primary use of `openshift-ansible` is to install, configure and upgrade OpenShift clusters.
+
+This is typically done by direct invocation of Ansible tools like `ansible-playbook`. This use case is covered in detail in the [OpenShift advanced installation documentation](https://docs.openshift.org/latest/install_config/install/advanced_install.html)
+
+For OpenShift Container Platform there's also an installation utility that wraps `openshift-ansible`. This usage case is covered in the [Quick Installation](https://docs.openshift.com/container-platform/latest/install_config/install/quick_install.html) section of the documentation.
+
+The usage examples below cover use cases other than install/configure/upgrade.
+
+## Container image
+
+The examples below run [openshift-ansible in a container](../README_CONTAINER_IMAGE.md) to perform certificate expiration checks on an OpenShift cluster from pods running on the cluster itself.
+
+You can find more details about the certificate expiration check roles and example playbooks in [the openshift_certificate_expiry role's README](../roles/openshift_certificate_expiry/README.md).
+
+### Job to upload certificate expiration reports
+
+The example `Job` in [certificate-check-upload.yaml](certificate-check-upload.yaml) executes a [Job](https://docs.openshift.org/latest/dev_guide/jobs.html) that checks the expiration dates of the internal certificates of the cluster and uploads HTML and JSON reports to `/etc/origin/certificate_expiration_report` in the masters.
+
+This example uses the [`easy-mode-upload.yaml`](../playbooks/certificate_expiry/easy-mode-upload.yaml) example playbook, which generates reports and uploads them to the masters. The playbook can be customized via environment variables to control the length of the warning period (`CERT_EXPIRY_WARN_DAYS`) and the location in the masters where the reports are uploaded (`COPY_TO_PATH`).
+
+The job expects the inventory to be provided via the *hosts* key of a [ConfigMap](https://docs.openshift.org/latest/dev_guide/configmaps.html) named *inventory*, and the passwordless ssh key that allows connecting to the hosts to be availalbe as *ssh-privatekey* from a [Secret](https://docs.openshift.org/latest/dev_guide/secrets.html) named *sshkey*, so these are created first:
+
+    oc new-project certcheck
+    oc create configmap inventory --from-file=hosts=/etc/ansible/hosts
+    oc secrets new-sshauth sshkey --ssh-privatekey=$HOME/.ssh/id_rsa
+
+Note that `inventory`, `hosts`, `sshkey` and `ssh-privatekey` are referenced by name from the provided example Job definition. If you use different names for the objects/attributes you will have to adjust the Job accordingly.
+
+To create the Job:
+
+    oc create -f examples/certificate-check-upload.yaml
+
+### Scheduled job for certificate expiration report upload
+
+**Note**: This example uses the [ScheduledJob](https://docs.openshift.com/container-platform/3.4/dev_guide/scheduled_jobs.html) object, which has been renamed to [CronJob](https://docs.openshift.org/latest/dev_guide/cron_jobs.html) upstream and is still a Technology Preview subject to further change.
+
+The example `ScheduledJob` in [scheduled-certcheck-upload.yaml](scheduled-certcheck-upload.yaml) does the same as the `Job` example above, but it is scheduled to automatically run every first day of the month (see the `spec.schedule` value in the example).
+
+The job definition is the same and it expects the same configuration: we provide the inventory and ssh key via a ConfigMap and a Secret respectively:
+
+    oc new-project certcheck
+    oc create configmap inventory --from-file=hosts=/etc/ansible/hosts
+    oc secrets new-sshauth sshkey --ssh-privatekey=$HOME/.ssh/id_rsa
+
+And then we create the ScheduledJob:
+
+    oc create -f examples/scheduled-certcheck-upload.yaml
+
+### Job and ScheduledJob to check certificates using volumes
+
+There are two additional examples:
+
+ - A `Job` [certificate-check-volume.yaml](certificate-check-volume.yaml)
+ - A `ScheduledJob` [scheduled-certcheck-upload.yaml](scheduled-certcheck-upload.yaml)
+
+These perform the same work as the two examples above, but instead of uploading the generated reports to the masters they store them in a custom path within the container that is expected to be backed by a [PersistentVolumeClaim](https://docs.openshift.org/latest/dev_guide/persistent_volumes.html), so that the reports are actually written to storage external to the container.
+
+These examples assume that there is an existing `PersistentVolumeClaim` called `certcheck-reports` and they use the  [`html_and_json_timestamp.yaml`](../playbooks/certificate_expiry/html_and_json_timestamp.yaml) example playbook to write timestamped reports into it.
+
+You can later access the reports from another pod that mounts the same volume, or externally via direct access to the backend storage behind the matching `PersistentVolume`.
+
+To run these examples we prepare the inventory and ssh keys as in the other examples:
+
+    oc new-project certcheck
+    oc create configmap inventory --from-file=hosts=/etc/ansible/hosts
+    oc secrets new-sshauth sshkey --ssh-privatekey=$HOME/.ssh/id_rsa
+
+Additionally we allocate a `PersistentVolumeClaim` to store the reports:
+
+	oc create -f - <<PVC
+	---
+	apiVersion: v1
+	kind: PersistentVolumeClaim
+	metadata:
+	  name: certcheck-reports
+	spec:
+	  accessModes:
+		- ReadWriteOnce
+	  resources:
+		requests:
+		  storage: 1Gi
+	PVC
+
+With that we can run the `Job` once:
+
+    oc create -f examples/certificate-check-volume.yaml
+
+or schedule it to run periodically as a `ScheduledJob`:
+
+    oc create -f examples/scheduled-certcheck-volume.yaml
+