configure registry in gstg to use the metadata DB

After confirming the registry DB setup in staging is complete (#1983 (closed)) we can configure registry in staging to make use of the metadata DB.

Config should look something like this:

registry:
  database:
    enabled: true
    host: pgbouncer-registry.int.gstg.gitlab.net
    user: gitlab-registry
    name: gitlabhq_registry
    password:
      secret: registry-postgresql-password-v1
    pool:
      maxopen: 8
      maxidle: 8
      maxlifetime: 5m
  migration:
    enabled: true
    rootdirectory: gitlab
  storage:
    secret: registry-storage-v3
  maintenance:
    uploadpurging:
      enabled: false