Allow test_l3_agent unit test to run individually

When the test_l3_agent.TestL3AgentEventHandler test case is run
after test discovery it passes because discovery imports files
that registers all the config options. For the test case to run
individually we need to explicitly register the config options
used in the test.

Change-Id: Ic31503ad7be8601ff843dbdc8edf5856ce8b0e39
Closes-bug: #1313812
Merge "Allow test_l3_agent unit test to run individually"