Docker bridge

Fixes #587 (Docker bridged networking mode)

For initial review.

- [x] Update docs
- [x] Update Mesos dependency once version 0.20.1 is released
- [x] Do dynamic global port assignments
- [X] Verify `ports` attribute works as expected
- [X] Publish and update dependency of mesos-utils for 0.20.1
What if `availablePorts` is empty?