Garbage collect HasStandardAttributes subclasses in StandardAttrTestCase
Garbage collect HasStandardAttributes subclasses in StandardAttrTestCase

Otherwise they may affect consequent calls to
get_standard_attr_resource_model_map on next plugin instantiation.

Change-Id: If8030776b3880e8c61980aeb28a65b397cca5ec4
Closes-Bug: #1625973

cloudbase/neutron,sebrandon1/neutron,openstack/neutron,mahak/neutron,cloudbase/neutron,mahak/neutron,sebrandon1/neutron,huntxu/neutron,noironetworks/neutron,noironetworks/neutron,openstack/neutron,eayunstack/neutron,openstack/neutron,huntxu/neutron,mahak/neutron,eayunstack/neutron