Allow tox tests with complex OS_TEST_DBAPI_CONNECTION URLs

In particular, this allows OS_TEST_DBAPI_ADMIN_CONNECTION URLs with
alternate drivers like mysql+mysqlconnector://... to survive through
into OS_TEST_DBAPI_CONNECTION for the actual unittest.

Change-Id: I712d9366e616a551f4b2bd225de42469a35b363f
Merge "Allow tox tests with complex OS_TEST_DBAPI_CONNECTION URLs"