Add 'rollback' command as allowed for failure_action

[shin-]

Updated #2091

[GordonTheTurtle]

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

$ git clone -b "NikolayMurha-master" git@github.com:docker/docker-py.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842354291488
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.