Test setup of repmgr in staging for HA postgres

As a trial by fire, lets try out the steps here: gitlab-org/omnibus-gitlab#1807 (comment 31216296) on staging to get a first impression of what HA postgres could look like in the future. This is a WIP, so we will be doing it manually (as discussed with @pcarranza, @ibaum and @marin) just so we can get a feeling for our solution, its limitations and possible effort estimations.