Container Registry: expose database configuration settings in Omnibus

Context

As we start experimenting with enabling the metadata database in Omnibus installations, we will need a way to permanently configure the Registry database settings.

Problem

During PoC: enable the metadata database for new insta... (#1065 - closed) it was noted that manual changes to the registry configuration are wiped by Omnibus if a gitlab-ctl reconfigure is run.

Also, these new database settings are still experimental, so we must clearly define and document them. Noting that we already expose these settings in the registry Helm chart configuration.

Solution

Expose the database settings in the Omnibus configuration and document them.