chore(contrib/gce): make GCE script pass flake8

Also do a few stylistic changes:

 - prefer double quotes instead of single quotes
 - functionize commonly done tasks
 - use exit code
chore(controller): update celery to 3.1.13

Several bug fixes, some specific to Django:
http://docs.celeryproject.org/en/latest/changelog.html#version-3-1-13
Merge pull request #1607 from mboersma/update-celery

chore(controller): update celery to 3.1.13
Merge pull request #1593 from deis/contrib/gce/flake8

chore(contrib/gce): make GCE script pass flake8