[WIP] Run ansible in a crontask on service hosts

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 took the line from the docs -- ``This can be a relative path within the checkout.`` -- for granted.