Remove unused context from test_sqlalchemy

Change-Id: I0b3c7341e91cfc57aed625754bc9bf009ef70b14
Remove pointless use of OpenStackException

The use of the base exception does nothing for the test but means
the "dead module walking" exception module.

Change-Id: Ibd7bb8aa526be366f005ab5e12fd4da89a976c57