Use metron agent by default, remove loggregator_endpoint properties

* ETCD is now used for metron_agents to find loggregator servers
* Loggregator TrafficController input proxy is no longer used; metron sends
  logs directly to loggregator servers
* Metron agent is co-located with other jobs, so those jobs should send logs
  to localhost

[#75821982]

Signed-off-by: John Tuley <jtuley@pivotallabs.com>