Improve Vagrant plugin's logging

[chungers]

This PR makes the vagrant plugin's stderr/stdout streamable to the logging output. This improves the overall usability of the plugin since there won't be a long delay when the code is trying to buffer up the entire command's output. It is now possible to watch in the console what vagrant is actually doing.

Signed-off-by: David Chung david.chung@docker.com

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/docker/docker/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "stream-vagrant-output" git@github.com:chungers/infrakit.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842353674872
editor opens
change each 'pick' to 'edit'
save the file and quit
$ git commit --amend -s --no-edit
$ git rebase --continue # and repeat the amend for each commit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.