Let users provide their own Ansible inventory

[noahl]

This lets users who already have an Ansible inventory not repeat themselves.

It also makes it possible to put inventories in version control. Once this feature lands, we should make an inventory file with our own test machines and put it in a private git repo to simplify testing.

[noahl]

For bonus points, support Ansible dynamic inventories as well. That would be a huge time-saver in some cases.