Add functional test for l3_agent

In order to insert the HA related code into the L3
agent, as part of blueprint l3-high-availability,
it's important to add functional tests for the L3 agent.

The L3 HA patch will use the framework provided here
to implement additional HA specific tests.

Implements: blueprint l3-high-availability
Change-Id: I49ddc95a0c41330580fcec6ba05c72684248af5e
Merge "Add functional test for l3_agent"