nuageinit: fix using user_data as a script

In official cloudinit, when a user_data file starts with '#!' it
should be execute late in the boot process. To respect this nuageinit
now copy the user_data script into a /var/cache/nuageinit/user_data if
found and a new "firsboot" rcscript anchored to the 'local' rc script is
responsible to execute it if found.

Note by doing this, we fix another issue we had with nuageinit, if the
cloudinit provider provides the user_data scriptout with the executable
permission, previous implementation was not working, like apparently
what Digital Ocean is doing.

PR: 287183
Reported by: olgeni@