tests.db: simplify connection handling
tests.db: simplify connection handling

We just make the managers responsible for returning an URL and possibly
create a database if needed, but that's it. The rest is now common and
handled by the base class.

Change-Id: I8118706aca9fedba9b6721c860db5805a7985a54

sileht/aodh,openstack/aodh,sileht/aodh,pczerkas/aodh,pczerkas/aodh,openstack/aodh