Prevent races in opportunistic db test cases

Opportunistic db test cases create schemas on demand, so that each
test case which inherits the base test case class, will get its own
db schema (i. e. races between tests are not possible).

In order to do schema provisioning we have to connect to RDBMS server
first. So far we've been connecting to the openstack_citest database,
which is guaranteed to exist on CI nodes. It turns out, there are a
few test cases in Nova (maybe in other projects as well), that drop
and recreate the openstack_citest database. If they happen to do that
when the opportunistic db fixture is in the middle of provisioning a
schema, those tests will fail (as there is an an open session to the
database and thus it can't be dropped).

This can be solved easily by changing the way we provision new
schemas in opportunistic db test cases as actually, we don't have to
connect to the openstack_citest database at all:

 - for MySQL we can use an empty db name to connect to MySQL server,
   but not to a particular database
 - PostgreSQL requires us to specify the database name. We can use
   the service postgres database here (PostgreSQL shell utils such
   as createdb, createuser, etc use it for the very same reason)

Closes-Bug: #1328997

Change-Id: I0dc0becc5cb40d3dab3289c865a96113522a0b9a