Auto PR new workflow #1008

Right now, each commit with changes to images create a PR to the official images repo. If however, a subsequent commit is merge here, a second PR is created for the official image containing the changes for both. To fix this here is my proposed new workflow

1. Check the existence of a branch called travis-node in the bot official image fork
2. If it doesn't exit, create it from upstream/master
3. If it does exist:
   1. Check if travis-node is behind or ahead of master
   2. If behind, reset travis-node to upstream/master
   3. If ahead, check if a PR is associated with it
   4. If no PR:
      1. Create a PR with the commit message as the title. Repeat the commit message in the body and cc the maintainers.
      2. Comment in the new PR the PR number in this repo to tag it
   5. If a closed PR exist reset travis-node to upstream/master
   6. If an opened PR exist:
      1. Commit the new changes on the travis-node branch and push
      2. Update the title to Multiple Node Updates
      3. Add the new commit message to the body
      4. Comment in the new PR the PR number in this repo to tag it

/cc docker-library/official-images#5514