metron picks trafficcontroller dynamically based on heartbeats in ETCD

[#74964832]

Signed-off-by: Johannes Petzold <jpetzold@pivotallabs.com>
automatic promotion by GoCD workflow LoggregatorIntegrationTest/76 > IntegrationTests/1 > IntegrationTests

tests passed against https://github.com/cloudfoundry/cf-release/commit/a37c9c2888f44a2fd751c82009a3a3c41d62fee7