Adapt DB provisioning code for CI requirements

With recent changes made to our CI MySQL openstack_citest user now
has enough priviliges to create and drop databases. But the code,
that we use for provisioning of databases must be updated, so that
it's not mandatory for openstack_citest user to be also able to
do GRANT/ALTER ROLE queries. Actually this simplifies provisioning
code a bit and allows to reuse the same queries for PostgreSQL and
MySQL.

This will be used for opportunistic db test cases as well as for
running of DbTestCase tests on different backends.

Change-Id: Ia917f647182d908dbb2a86659ad8b4e49c492881
Merge "Adapt DB provisioning code for CI requirements"