Adds L3 agent test case to test_l3_agent_scheduler.py

This submission is to verify "add L3 Agent" and "remove L3 Agent"
functionality and also added required methods in both XML/JSON
interface

Change-Id: I7829b7aefdea3c138ffb8070ba1497343f4a11fe
Merge "Adds L3 agent test case to test_l3_agent_scheduler.py"