fix confusion between useradd and adduser with --disabled-password

It seems there was a confusion between `useradd` and `adduser` as `useradd` doesn't take the --disable-password and --disable-login options. It was causing an error when trying to create a user with these options, at least on Ubuntu.

I solved the issue by switching to a default:

    $ adduser --disable-password --gecko "" <username>

with `disable_password=False`, the user will be prompted with password input