OpenShift: Provide example files and documentation

Summary

While the Operator works out of the box on OpenShift, the GitLab chart needs some customization (mostly RBAC and security context related) to run on OpenShift. We should provide example values and general documentation on our OpenShift support.

The only OpenShift related docs we have are cluster setup related and some examples for GitLab shell.

Acceptance Criteria

• Provide an example values for to install the chart on OCP.
• Document how we do (not) OpenShift.