Ensure networking is up before starting docker

This resolves a problem that I have been having where docker starts before networking is up. See issue #5944 for more details.

Docker-DCO-1.1-Signed-off-by: Jeffrey Bolle <jeffreybolle@gmail.com> (github: jeffreybolle)
Merge pull request #6029 from jeffreybolle/master

Update upstart script to ensure networking is up before starting docker