Clean shutdown of infrakit plugin start --wait

[chungers]

The utility infrakit plugin start in wait mode (--wait) will wait for the plugins it started to shutdown before terminating. This PR fixes a bug where it hangs even when all the plugins
have been stopped by another infrakit plugin stop --all.

[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 "0.3.0-proper-cleanup" git@github.com:chungers/infrakit.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842354379296
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.