Retrieve path to Postgres config directory from server

Currently the path is hardcoded for Postgres 9.4.

This works when using Ubuntu 14.04 as base image, but Ubuntu 14.04.3 (now on Digital Ocean) ships with Postgres 9.5

Solution:
Gather Postgres version as 'fact' and set the config path to include the current version number.

This will make this way more robust and possibly allow the use of all recent Ubuntu images as base.
