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.
I've been thinking about the shell service account should have.

By default, it's gonna be `sh`.