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
I will be able to work on fixing the juju bug about private/publi adress when the demos will be usable hence pending your merge .. :)