Automatic periodic stack refresh

[imjasonh]

In order to ensure VM images are up-to-date, we should periodically refresh the stack, basically by running setup.sh. This would be possible a few ways:

1.) Have a GAE Flex cron job that literally runs setup.sh
2.) Have a GAE Standard cron job that runs a Go program equivalent to setup.sh
3.) Have a GAE Standard cron job that begins a Deployment Manager deployment (See #5)