Re-use common methods in test_delete_server_while_in_attached_volume

This changes the test_delete_server_while_in_attached_volume test
to re-use common volume create/attach methods in the base compute
API test class.

Note that this changes the volumes client used to create, get and
delete the volume. The test was previously using the proxy volumes
extension from the compute API which is now deprecated.

Change-Id: Ief13602a6764c3259a827d6d2a4c40436f04667a
1 file changed