Added user service option in case you run postgres with a not default user

Sometimes you want to run postgres with a non default user (`postgres`), for example `nobody` like is my case because I start postgres with supervisord.

I added 2 more vars:

    postgresql_admin_user
    postgresql_admin_group

By default they work like always, they use `postgresql_admin_user` and this by default `postgres`, so the default behaviour is `postgres` user