Tries to advertise valid default IP

During the first heartbeat, the heartbeater asks the agent to check
its advertised address; if the advertised IP is still the default
(None), the agent tries to replace it with the IP of the first network
interface it finds.  If it fails to find either a network interface or
an IP address, the agent raises an exception.

Change-Id: I6d435d39e99ed0ff5c8b4883b6aa0b356f6cb4ae
Closes-Bug: #1309110
Merge "Tries to advertise valid default IP"