quantum security group driver nova list shows same group

When using the quantum security group driver nova list shows the
same security group for all instances. This patch fixes this and
corrects the unit test that was hiding this bug. This also adds a unit test
to check that the default security group is added to the response when
not specified.

Fixes bug  1162077

Change-Id: I7a10f81af1bb6c93f634dc0de27841afe0f1f0ea