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
Better to use hostname --all-ip-addresses rather than the ip route | grep src | cut -d " " -f 12? Seems a bit fragile?  Applies throughout.