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
out of memory there is 1 case (like private-adress only) where val_ip can be empty but not val ... well it has been tested :)