nuageinit: only create the default user when needed

The "default" user should only be created when:

• the users key is missing
• or the default string is present in the users list

Since the public_keys is extracted from the meta-data, this patch has
to slightly adjust the way they are loaded.
The change simplify the logic around the default user SSH key injection.
Both ssh_authorized_keys and public_keys are handled at the same time.

MFC After: 1 week
Signed-off-by: Gonéri Le Bouder <goneri@lebouder.net>
Pull Request: https://github.com/freebsd/freebsd-src/pull/1952
(cherry picked from commit cbc00fcc2b92e6e38b0a180261547b1a22b461bd)