Stop using mox stubs in nova/tests/unit/test_metadata.py
Stop using mox stubs in nova/tests/unit/test_metadata.py

Replace 'stubs.Set' with 'stub_out' in
nova/tests/unit/test_metadata.py.

Change-Id: I73fb5b5dc32ec883b77f86676d45c916e2185ab7
Implements: blueprint remove-mox-newton

jianghuaw/nova,rajalokan/nova,gooddata/openstack-nova,rajalokan/nova,alaski/nova,hanlind/nova,Juniper/nova,jianghuaw/nova,rahulunair/nova,hanlind/nova,mikalstill/nova,gooddata/openstack-nova,rajalokan/nova,vmturbo/nova,openstack/nova,hanlind/nova,phenoxim/nova,Juniper/nova,klmitch/nova,gooddata/openstack-nova,jianghuaw/nova,Juniper/nova,klmitch/nova,vmturbo/nova,mahak/nova,sebrandon1/nova,mahak/nova,gooddata/openstack-nova,cloudbase/nova,rahulunair/nova,vmturbo/nova,sebrandon1/nova,mikalstill/nova,vmturbo/nova,jianghuaw/nova,cloudbase/nova,cloudbase/nova,openstack/nova,klmitch/nova,mahak/nova,rajalokan/nova,phenoxim/nova,mikalstill/nova,alaski/nova,Juniper/nova,openstack/nova,rahulunair/nova,sebrandon1/nova,klmitch/nova