GlusterFS: Various unit test improvements
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

petrutlucian94/cinder,Nexenta/cinder,bswartz/cinder,apporc/cinder,dims/cinder,winndows/cinder,alex8866/cinder,eharney/cinder,github-borat/cinder,Paul-Ezell/cinder-1,Akrog/cinder,mahak/cinder,openstack/cinder,julianwang/cinder,petrutlucian94/cinder,phenoxim/cinder,Datera/cinder,blueboxgroup/cinder,hguemar/cinder,nexusriot/cinder,nexusriot/cinder,openstack/cinder,rakeshmi/cinder,takeshineshiro/cinder,potsmaster/cinder,blueboxgroup/cinder,Hybrid-Cloud/cinder,sasukeh/cinder,Accelerite/cinder,CloudServer/cinder,julianwang/cinder,phenoxim/cinder,tlakshman26/cinder-https-changes,cloudbase/cinder,scottdangelo/RemoveVolumeMangerLocks,duhzecca/cinder,rakeshmi/cinder,mahak/cinder,Paul-Ezell/cinder-1,github-borat/cinder,Datera/cinder,nikesh-mahalka/cinder,takeshineshiro/cinder,NetApp/cinder,Akrog/cinder,scottdangelo/RemoveVolumeMangerLocks,saeki-masaki/cinder,abusse/cinder,winndows/cinder,tlakshman26/cinder-bug-fix-volume-conversion-full,JioCloud/cinder,j-griffith/cinder,nikesh-mahalka/cinder,sasukeh/cinder,tlakshman26/cinder-bug-fix-volume-conversion-full,scality/cinder,bswartz/cinder,Nexenta/cinder,manojhirway/ExistingImagesOnNFS,cloudbase/cinder,tlakshman26/cinder-new-branch,hguemar/cinder,potsmaster/cinder,duhzecca/cinder,j-griffith/cinder,leilihh/cinder,Hybrid-Cloud/cinder,CloudServer/cinder,apporc/cinder,JioCloud/cinder,tlakshman26/cinder-https-changes,abusse/cinder,scality/cinder,manojhirway/ExistingImagesOnNFS,redhat-openstack/cinder,tlakshman26/cinder-new-branch,NetApp/cinder,alex8866/cinder,dims/cinder,leilihh/cinder,redhat-openstack/cinder,Accelerite/cinder,saeki-masaki/cinder,ge0rgi/cinder,eharney/cinder