Make multi_host configurable, add support for the nrpe-external-master subordinate.
Add workaround for when the mon_hosts relation returns host names and not IP addresses.
Use get_ip function in place of give_me_numbers.
Remove give_me_numbers as it is no longer needed.
mon_host workaround, optional multi_host, nrep-external-master sub.

Merged lp:~aglenyoung/charms/precise/nova-compute/optional-multi_host.