There are some issues in test_volume.py in volumev2,
I make three changes in this patch:
1.modified some codes that not used FakeVolume class.
2.added a fake image by FakeImage class for this test.
3.added a fake snapshot by FakeSnapshot class for this test.
Change-Id: I02ba73d3aaee95624b0e2307b255e0e485b0c3a3