Add support for configuring Omniauth

Incorporate support for configuration of Omniauth within the Unicorn pod.

I've tried to keep this as simple and flexible a change as possible. I have included documentation on the use, with pointers to our public documentation in regards to providers/support/syntax where applicable.

The concept is to take gitlab.unicorn.omniauth.providers array, map that into the configuration file, while keeping the secrets in Kubernetes Secrets that are populated as a part of configure container. The secrets are included in the Deployment via the same logic that handles their population in the ConfigMap.

These secrets can not be added via the shared-secrets pod, as their sources come from outside the environment.

Closes #335 (closed)

I've tested this by deploying to gitlab.separate-containers.party. Note, that I have Omniauth on, but have not enabled auto-creation of accounts from it. So for verification, you will need to register, then associate your appropriate account.

I am still evaluating the requirement to include this with our CI deployment.