Allow to use socket to connect to DB in Nailgun

Right now it is possible to specify a unix-socket in settings
as a way to connect to the PostgreSQL database. It must be specified
as the 'host' settings:

DATABASE:
  engine: "postgresql"
  name: "nailgun"
  host: "/var/run/postgresql"
  port: ""
  user: "nailgun"
  passwd: "nailgun"


DocImpact
Closes-Bug: #1398728
Change-Id: Iaabd455edcf1311b162ceb3403879313e6e67825
Merge "Allow to use socket to connect to DB in Nailgun"