fup: Move _wait_for_volume_{attach,detach} to os-volume_attachments
fup: Move _wait_for_volume_{attach,detach} to os-volume_attachments

As documented in the api-ref this is the API end users would poll to
determine the state of a volume operation so use it in our func tests.

Change-Id: I88e827603bb1cd6265bf234f4286ba3b204c3ce1

openstack/nova,mahak/nova,openstack/nova,mahak/nova,openstack/nova,mahak/nova