Add possibility to handle remote managed postgresql setup

This is to address another part of issue #6 (closed)

This has been only partly tested (on the new possibility of remote postgresql setup), on classic (remote) Ansible deployment, but not through the shell script + ansible mix installation.

The new keys have been set in order to match current expectations (i.e. a local install of postgresql).

I end up declaring funkwhale_database_url in playbook and modifying DATABASE_URL in .env (it feels redundant).