L3 agent fetches the external network id once

L3 agent fetches the external network id once

Rather than fetching the id of the external network each time that
_process_routers is called, get it once and remember it.  If the agent
is ever requested to connect to a different ext-net then it will fetch
the current ext-net to double check for the unlikely event that the
ext-net has changed.  If it has then it will remember the new ext-net.

This is only applicable in the case where there is only one ext-net
that has not been configured explicitly in the config file.  That was
the only case that would cause an RPC message in the first place.

Change-Id: I40bbbf6233131ea5d40122ef9495fd3cb7dc823a
Closes-Bug: #1269567