Improve test coverage: volume attach/detach API
Improve test coverage: volume attach/detach API

Add missing tests for attaching and detaching volumes to and from
servers.

Change-Id: I0fa9f9b0190cbd31d4c9b0cc6422c2f876510f77

stackforge/python-openstacksdk,openstack-infra/shade,openstack/python-openstacksdk,dtroyer/python-openstacksdk,dtroyer/python-openstacksdk,stackforge/python-openstacksdk,openstack/python-openstacksdk,openstack-infra/shade