Expand PgBouncer and Consul HA support to handle multiple databases in the same cluster

As it stands today HA failover support, achieved through a combination of PgBouncer and Consul, in omnibus is supported only for the main GitLab database - gitlabhq_production.

With the addition of Gitaly Cluster and Praefect however it requires a separate database. For efficiency and maintenance reasons the desire will always be to have the databases on the same database cluster. However this isn't currently possible as failover is only handled by PgBouncer and Consul for the gitlabhq_production database and there doesn't appear to be a way to get it to track other databases on the same cluster.

Without this capability we can't use Praefect on the same database cluster, which is a notable inconvenience for Customers as well as ourselves. From previous interactions with customers I believe the general consensus will strongly be that they'll want to use the same cluster where possible.

Specifically adding this ability to make it so PgBouncer and Consul can track other databases will unlock this and any other future components to be fully managed in Omnibus.