Convert mox to mock: tests/compute/test_nova.py

Replace mox testing library by mock in the file
cinder/tests/compute/test_nova.py

Implements: blueprint mox-to-mock-conversion
Change-Id: Ic819afb4391708a0afebe4fc2855c93e3cb3f4c4
Merge "Convert mox to mock: tests/compute/test_nova.py"