The Openstack metadata service exposes the EC2 API. We use the
existing `ec2.nix` module to configure the hostname and ssh keys of an
Openstack Instance.

A test checks the ssh server is well configured.

This is mainly to reduce the size of the image (700MB). Also,
declarative features provided by cloud-init are not really useful
since we would prefer to use our `configuration.nix` file instead.

This allows the VM to provide a `configuration.nix` file to the VM.

The test doesn't work in sandbox because it needs Internet (however it
works interactively).

People don't necessary know `nova` is related to Openstack (it is a
component of Openstack). So, it is more explicit to call it
`openstackImage`.

This is to let the `ec2-data.nix` module sets the hostname from the
metadata API value.

To share the metadata fetcher script between ec2 and Openstack images.

Remove cloud-init from the Openstack image configuration