GlusterFS tests: Mock out compute, don't load novaclient

Mock out the compute library for these tests, which prevents loading
the novaclient library.

This makes these tests more reliable, as currently the novaclient
lib interferes with our mocking of os.path.exists in these tests
in some test configurations.

This should also make the GlusterFS tests slightly more efficient
since they were previously loading the novaclient python library
but did not really need to.

Change-Id: I49e858b8536e30572b6231f209a35578460fff96
Merge "GlusterFS tests: Mock out compute, don't load novaclient"