Specify exactly how and where Ansible runs (bootstrap, production crontask, development)

This should help illustrate my mailing-list conversation wondering exactly how we should accomplish this.

Because this won't work well in development (it will always look at
prod-hosts), it's easily disabled with no_ansible_pull=true in the
inventory.  Fixes #3089.
Right. Default variables are put in `defaults/main.yml`. These are loaded last, making them easy to override.