Make postgres work on Jenkins slaves again.

A few upstream puppetlabs postgres module bugs need to be worked around
in order to have postgres servers installed on our slaves. Need to
create the postgres user and group before the upstream module runs and
provide hostnames to the postgres server config instead of ip addresses.

Change-Id: I68cea595d0cd6ad9b2155166ff473d1375e9b7ac
Merge "Make postgres work on Jenkins slaves again."