Adds delete_volume_attachment method to behaviors for attach api.

 * New method waits for volume attachment to return a 404 after
   issuing a delete.
 * Minor pep8 cleanup of the behaviors module.
 * Cleaned up some error message formatting.

Change-Id: I33d89d17dd900d18677b8a8c7b58d7432111dfc5
Merge "Adds delete_volume_attachment method to behaviors for attach api."