Allow a Secret to be used in application container

What does this MR do ?

This creates an option where users can pass in a setting to specify the name of the Secret. This name will be used for deployments to load the Secret's key-value pairs as environment variables in the application container.

This option is optional, hence this MR is backwards compatible. The Secret must be created by the user beforehand.

Issue

https://gitlab.com/gitlab-org/gitlab-ce/issues/49056

Links / References

• secretRef is available as far back as 1.7 https://v1-7.docs.kubernetes.io/docs/api-reference/v1.7/#envfromsource-v1-core