ext-gw update on dvr router improperly handled by l3-agent

The recently added external_gateway method does not check
the agent mode or host binding before adding an external
gateway to a particular node/agent.  This fix adds the
required checks in the update path.

Change-Id: Idbb76eea1faec1c5d6b7615d23d74760cc327bb2
Closes-bug: #1356734
Merge "ext-gw update on dvr router improperly handled by l3-agent"