PR Plugin now uses service_name to identify hosts

Now the Physical Resource plugin uses the service name instead of
hypervisor host name. This is because in Havana (and could be
a chance that happens in Icehouse), the host name of the compute node
is not the same as the service name.

Change-Id: Id958f57539d2e3bcbc93698cb6151dcedfcb549f
Closes-Bug: #1301294