Refactor volume clone tests
Refactor volume clone tests

Creating a common method 'volume_clone_verification' in order to
reduce a duplicated code.

Change-Id: If73ab72d0ce3218a6a3ba8b5a5c812507d800a21

openstack/tempest,masayukig/tempest,openstack/tempest,Juniper/tempest,masayukig/tempest,cisco-openstack/tempest,vedujoshi/tempest,Juniper/tempest,vedujoshi/tempest,cisco-openstack/tempest