Tests: fix quobyte breaking other tests
Tests: fix quobyte breaking other tests

This patch fixes the Quobyte driver unit tests that forcefully replace
the 'convert_image' method in 'image_utils' with a Mock and then doesn't
restore it afterwards, so any method that runs afterwards and expects
'convert_image' to actually run code will fail.

This patch uses the proper mocking mechanisms that restore things
afterwards.

Change-Id: Iec1cf6ade1d97900d4db6e29555ddef8d45b9534

openstack/cinder,openstack/cinder,mahak/cinder,mahak/cinder