Fix sql_upgrade tests run by themselves

The tests in test_sql_upgrade would fail to run individually, with
a None dereferencing error:

 Traceback (most recent call last):
   File "keystone/tests/test_sql_upgrade.py", line 135, in setUp
     conn_str.startswith('sqlite') and
 AttributeError: 'NoneType' object has no attribute 'startswith'

Change-Id: Id05ec05a344abab8d588a4761c207e2db715e750
Merge "Fix sql_upgrade tests run by themselves"