Role postgresql, task systemd: factorize tasks.

This is an improvement to the systemd.yml file which had a lot of tasks doing the same thing, with slighty different names for the unit files.

I factorized the 'start postgresql.service' tasks so that we only need one task with a dynamic unit file name.

Remarks: This means that we no longer have system-specific tasks. It doesn't matter in the current state as there was nothing system-specific in our tasks.