gitlab: Wait a bit for PostgreSQL to be ready in Unicorn startup

As it was outlined in 5a744de7 (gitlab: Compile assets on instantiation
and make sure DB is properly setup/migrated before unicorn runs) we are
performing DB initialization and migration in pre-action as part of
unicorn startup script. But that currently has one drawback:

    if all services start at the same time - e.g. both PostgreSQL and
    Unicorn - and that is a common scenario when SR is compiled and
    instantiated / started, PostgreSQL is usually not yet ready to
    process queries from Unicorn startup script, and first-time Unicorn
    startup fails.

Until now this problem was workarounded by manually starting unicorn
second time - after some time postgresql is started and ready. But why
do it manually, if we can do the same logic automatically. So fix it:

    in Unicorn startup script wait a bit (up to 5 seconds) for
    PostgreSQL to become ready.

/cc @kazuhiko, @jerome