Using the Ansible OpenStack inventory plugin, we can automatically fetch
information about running instances in HostVDS. We're deriving group
membership from the `groups` metadata tag.
The OpenStack API password must be specified in a `secure.yaml` file.
We're omitting this from the repository because there's no apparent way
to encrypt it.
The inventory plugin tends to prefer IPv6 addresses over IPv4 when
populating `ansible_host`, even if the control machine does not have
IPv6 connectivity. Thus, we have to compose the relevant variables
ourselves with a Jinja2 expression.
