Enables the ability to configure the notification_secret separate from the Geo configuration

What does this MR do?

On GitLab.com, there are times where we configure the notification_secret to enable statistics into SnowPlow. This MR enables us to create the necessary configuration in the gitlab.yml that is consumed by our webservice Pods such that the secret they require can be utilized.

We do this via utilizing the existing notification_secret that was originally intended for Geo only. We will now automatically create the necessary secret if a user chooses not too. Due to not being able to detect if the secret exists, we provide a warning message to the user prior to install in the case that the shared-secrets chart is not enabled.

Testing

Prior to this MR, the Secret object <release-name>-registry-notification was not created automatically. This MR will either allow one to specify the desired secret, or rely on the shared-secrets chart to do so.

Using a minimal values.yaml file, the above secret will be created, and it'll be populated at production.registry.notification_secret within gitlab.yml of Rails pods.

Testable via helm upgrade a ./ --install --values examples/minikube.yaml

And then hopping onto any pod that runs rails and observing the /srv/gitlab/config/gitlab.yml configuration file looking for the aforementioned key.

Related issues

Closes: #2653 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened