Return no active network if the agent has not been learnt yet

Sometimes the agent message to get the active networks is
processed before the agent is learnt by Neutron. Neutron throws an
error, but it should simply return no network

Change-Id: I6da0ca578cfe1fe0885138a5e2da76278f791491
Closes-bug: #1302838
Merge "Return no active network if the agent has not been learnt yet"