external_network should auto-detect all external networks or allow for list of nets

[Drew Freiberger]

As part of the openstack dashboard, the template renders external_network (= ext_net by default) into the template to determine number of used/free floating IPs.

This dash template should be altered to allow for either zero external networks, or multiple external networks to be defined to show available fips on all nets, or provide openstack dash that's workable for clouds w/out an external net (not likely to exist, but would be nice to get stats before properly configuring external_network on the charm).