Comment 2 for bug 1674553

There are two aspect, port binding and agent liveness report.
Regarding to port binding, in networking-odl, there is websocket logic and prepopulate logic. with them we don't use polling loop to update agentdb.

Regarding to agent liveness, ml2 driver has agent health check by timer.
self.add_agent_status_check_worker(self.agent_health_check)
it causes annoying log message warning agetn is dead.
and via agent api, cloud admin can get a list of agent. the admin will be surprised by many (wrongly) dead ODL pseudo agent.