Configure driver for postgres

Avoid the following warning seen in e.g. nova unit tests:

  WARNING oslo_db.sqlalchemy.engines [-] URL postgresql://.../foo does
  not contain a '+drivername' portion, and will make use of a default
  driver.
  A full dbname+drivername:// protocol is recommended.

The 'psycopg2' driver is chosen since this is the default for SQLAlchemy
[1] and therefore what we're currently using.

[1] https://docs.sqlalchemy.org/en/14/core/engines.html#postgresql



Change-Id: I0f8f358a49a83366d7dec17e7bac346453f3027b
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>