Update L3 agent drivers singletons to look at new agent

L3 agent drivers are singletons. They're created once, and hold
self.l3_agent. During testing, the agent is tossed away and
re-built, but the drivers singletons are pointing at the old
agent, and its old configuration.

Change-Id: Ie8a15318e71ea47cccad3b788751d914d51cbf18
Closes-Bug: #1404662
Merge "Update L3 agent drivers singletons to look at new agent"