The issue:
In its previous state the task directly edited postgresql's systemd unit file.
This file is systematically replaced by each postgresql update.
This is fine as long as we roll the update with this role only, but will break
as soon as an update is rolled by any other mean.

The solution:
Create an override file located in /etc/systemd/system/postgresql.service with
the appropriate custom options for the unit file (what we used to write in the
unit file).
This file will not be overriden and is therefore more stable.