Allow biboumi to start without an xmpp server to connect to

What happens

It fails to start because it can't connect to the configured XMPP server.

What I expect

Biboumi starts and wait for the XMPP server to appear. Polling every so often.

I am not sure why this restriction was placed here in the first place, this looks like an administrative burden. Imagine biboumi and the XMPP server being enabled, if biboumi starts first and fails, as the contrib .service file can't depend on a server, (which one?), the admin has to manually restart it.