Removing unnecessary setUp()/tearDown() in SecurityGroupsTestCase

Removing unnecessary setUp()/tearDown() in SecurityGroupsTestCase

This patch removes not required setUp()/tearDown() functions in
SecurityGroupsTestCase. The functions are almost identical in
test_db_plugin.QuantumDbPluginV2TestCase. Only difference is initializing
ext_mgr which is done by SecurityGroupDBTestCase.setUp().

And redundant inheritance of unittest2.TestCase for SecurityGroupsTestCase
is fixed.

Change-Id: I50d8efd047e23d02d76b8f38b75c730b75bb96ce