inventory: Configure for HostVDS openstack

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.

hostvds.openstack.yml

@@ -0,0 +1,18 @@
+plugin: openstack.cloud.openstack
+only_clouds:
+- hostvds
+cache: true
+cache_plugin: jsonfile
+cache_connection: .inventory-cache
+compose:
+  ansible_user: '"root"'
+  ansible_host: &ipv4
+    (openstack.addresses[openstack.addresses|first]
+      | selectattr('version', 'eq', 4)
+      | first
+    ).addr
+  ansible_ssh_host: *ipv4
+keyed_groups:
+- key: openstack.metadata.groups|split(',')
+  prefix: ''
+  separator: ''