Ensure local Postgres is started before attempting migrations

Potentially a bit of an edge case here but we (Quality) were stung by this recently.

As part of a test task to check if GitLab could be run on 2GB of RAM we added in a stop command for all GitLab components during an update to release some memory for that process. Through that we hit an issue where whenever database migration were subsequently run on further updates they would always fail as Postgres is never started.

We may not expect users to specifically issue a stop for Postgres locally but with the omnibus scripts my expectation was they would've still handle this potential scenario.