nixos/cloudstack-image: initial import

Cloudstack images are simply using cloud-init. They are not headless
as a user usually have access to a console. Otherwise, the difference
with Openstack are mostly handled by cloud-init.

This is still some minor issues. Notably, there is no non-root user.
Other cloud images usually come with a user named after the
distribution and with sudo. Would it make sense for NixOS?

Cloudstack gives the user the ability to change the password.
Cloud-init support for this is imperfect and the set-passwords module
should be declared as `- [set-passwords, always]` for this to work. I
don't know if there is an easy way to "patch" default cloud-init
configuration. However, without a non-root user, this is of no use.

Similarly, hostname is usually set through cloud-init using
`set_hostname` and `update_hostname` modules. While the patch to
declare nixos to cloud-init contains some code to set hostname, the
previously mentioned modules are not enabled.