Cleanup for local variable in test_attach_volume

This patch cleanup the test_attach_volume for unnecessary and
unused local variable.

During https://review.openstack.org/#/c/87191/7/, it came out
that we should clean up this file for unnecessary local variable.
So for logical breakdown of changes, this patch do the cleanup
part separately.

Change-Id: I4afc88a09f43f8b068b16368cbddb4bff21059c6
Merge "Cleanup for local variable in test_attach_volume"