Use single db_host when multi postgresql::listen_adresses

Fixes #1166 (closed)

Postgresql allows to listen on multiple adresses: https://www.postgresql.org/docs/9.5/static/runtime-config-connection.html#listen_addresses

ActiveRecord accept one host for a connection (seems legit)

This commit allow user to specify in ['gitlab']['postgresql']['listen_address'] and ['gitlab-ci']['postgresql']['listen_address'] multiple addresses, comma-separated, as accepted by Postgresql.

In parsing time, it take as db_host the first ip from the list.

cc @marin