Fix tests for ServerContraint
Fix tests for ServerContraint

The test case for ServerConstraint was using the wrong parent class,
since it was not using anything from its setUp.
This small improvement avoids creating unnecessary mocks and stubs
by using the HeatTestCase as parent directly.

Change-Id: I8a34967e76c82d2c7755b0c0fe83038071e1b0f7
cryptickp/heat,miguelgrinberg/heat,steveb/heat,rh-s/heat,dims/heat,srznew/heat,maestro-hybrid-cloud/heat,pshchelo/heat,dragorosson/heat,gonzolino/heat,openstack/heat,pshchelo/heat,dragorosson/heat,pratikmallya/heat,cwolferh/heat-scratch,rh-s/heat,openstack/heat,gonzolino/heat,maestro-hybrid-cloud/heat,dims/heat,steveb/heat,jasondunsmore/heat,noironetworks/heat,cwolferh/heat-scratch,miguelgrinberg/heat,noironetworks/heat,jasondunsmore/heat,rdo-management/heat,pratikmallya/heat,rdo-management/heat,takeshineshiro/heat,cryptickp/heat,srznew/heat,takeshineshiro/heat