[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.
https://github.com/ansible/ansible/blob/devel/bin/ansible-pull#L177
```
 cmd = 'ansible localhost -i "%s" %s -m %s -a "%s"' % (
inv_opts, base_opts, options.module_name, repo_opts
)
```
(in the copy I'm running, that's actually `ansible all`)

If you pass `-i prod-hosts` to `ansible-pull`, then it tries to use that inventory file for both the checkout and the playbook run.  It doesn't work for the former, but is required for the latter.