nuageinit: add basic support for cloudinit.

this is a very early script to support cloudinit, it does not intend to
be a full featured cloudinit client, but will support a good enough
subset to be viable in most case.

It support nocloud and openstack config-2 config drive mode (iso9660 or
msdosfs)

The following features are currently supported:

• adding users (including a default user named 'freebsd' with password 'freebsd'
• adding groups
• adding ssh keys
• static ipv4, static ipv6, dynamic ipv4

With this one is able to use the 'bring your own image feature" out of
box.

It is expected that the script grows the support of other clouds
supporting cloud-init, contributions are welcomed.

It is designed to be only run once via the firstboot mecanism.

Sponsored by: OVHCloud
MFC After: 3 weeks
Differential Revision: https://reviews.freebsd.org/D44141