Use pypowervm fixture for media UT
Use pypowervm fixture for media UT

Change test_media.py to use pypowervm UT fixture.
Update test case to use a property mock rather than our own class.

Change-Id: I475476b5f2104836eafbf1d65b478d6120acbcad

stackforge/nova-powervm,openstack/nova-powervm,openstack/nova-powervm,stackforge/nova-powervm