Last modifications to work on OpenStack

As been tested with this : http://code.launchpad.net/~thomnico/charms/precise/dns/openstack-support

Worked on both EC2 and private openstack. Tested with sipml5 charm in the charmstore.

Please comment
This is contrary to Metaswitch@245ae6c. Rob suggests that it should be beneficial in some networks (and benign in others).  Can we leave this in?