feat(logspout): introduce deis/logspout

In Deis' scheduler abstraction, we run a "sidecar" unit which tails the logs from each application and ships them to deis/logger.

This has been refactored into a micro-service image called deis/logspout, taking inspiration from [progrium/logspout](/progrium/logspout).

Logspout listens directly to a docker socket bind-mounted into the container and listens to the docker events API for running containers. Logs from running containers on the same host are shipped to deis/logger for further processing. This allows us to remove the hard dependency on running a sidecar container. Instead, one container per host ships all logs running on that host to deis/logger.

closes #1714 
closes #732 
closes #1311 

TODO:

- [x] fix unknown issue with connecting to etcd (container attempts to connect to etcd which is up, but the etcd client returns `501: All the given peers are not reachable (Tried to connect to each peer twice and failed)`)
- [x] add documentation
- [ ] add deis/logspout to jenkins automated builds
@bacongobbler need to add back sponsor.