GlusterFS: Various unit test improvements

Fixed bug that "_ensure_share_writable" was not mocked and therefore
"GlusterfsException" was thrown earlier than expected with an unexpected
error message.

To better guard against situations like this, I've added a helper method
"assertRaisesAndMessageMatches" to test for the thrown exception AND its
message.

Added "mox.VerifyAll()" where it was missing and cleaned up unused
mocks.

Fixed test for "create_volume_from_snapshot" (which actually tested
"create_cloned_volume").

Added tests for driver function "copy_volume_to_image".

Overall, code coverage for volume/drivers/glusterfs.py improved from 74%
to 82%.

Change-Id: I872eb477351c5406d07bbf5b748ebde2d878120a
Merge "GlusterFS: Various unit test improvements"