fixes nova_floating ip

* updates to nova_floating_ip:
  * delete floatingip
  * unassociate floatingip if instance is not found
  * cleanup
* updates to get_nodes to support nova
* refactor changes to work w/ oslab
* introduce retries to floatingip, ping, ssh

Change-Id: Id3d01f98cbf2027d7776df1cbfff1508c6d27b84