docker watcher proposal: identify services by label instead of port and image name

[dnephin]

Docker 1.6 added labels for images and containers.

Instead of requiring the same port for every service and exposing the port to the host (#109, #88), read a com.airbnb.synapse.service-port label from the image or container, and register the service using that port.

Another option might be to keep specifying the port in the watcher config, but look for a com.airbnb.synapse.service label.

Containers without a label would not be registered.