Option for Running and Testing Against Pre-populated Postgres

[jmchilton]

https://github.com/bgruening/docker-galaxy-stable/tree/master/compose/galaxy-postgres

[mvdbeek]

Galaxy skips the migrations for a while now, and with #985 and #986 this works now in planemo as well, so I think this isn't needed anymore.