nuageinint: implement ssh_pwauth

ssh_pwauth sets the value in sshd_config for the password authentication
This implementation tries to avoid touching the file if cloudinit
request for what is already the default value.

MFC After: 3 days
Sponsored by: OVHCloud
Reviewed by: kevans, jlduran
Differential Revision: https://reviews.freebsd.org/D49875