nuageinint: implement ssh_pwauth

[bapt]

Summary

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.

Sponsored by: OVHCloud

[emaste]

Looks fine to me. I'll tag @kevans for Lua review.

libexec/nuageinit/tests/nuageinit.sh

497

510

[bapt]

address @emaste comment

[jlduran]

Pedantically, to be on the same page in sshd_config(5):

1. The first configured value is the one that counts.
2. Some OSs (macOS)/Linux distros (Debian-based) just add Include /etc/ssh/sshd_config.d/* at the very top of the configuration file, so cloudinit should just create a new file under this directory with the desired settings, and call it a day. I only mention this option, as it is becoming popular (in case anyone has friends in openssh-portable ;-).
3. I would have rather/also liked to see something like:

# ssh_pwauth: true
printf "#PasswordAuthentication no # Should be ignored\n" > etc/ssh/sshd_config
printf "PasswordAuthentication no # Should change\n" >> etc/ssh/sshd_config
atf_check -o empty -e empty /usr/libexec/nuageinit "${PWD}"/media/nuageinit nocloud
atf_check -o match:"^#PasswordAuthentication no # Should be ignored$" \
    -o match:"^PasswordAuthentication yes$" cat etc/ssh/sshd_config

Regardless, the implementation seems correct.

libexec/nuageinit/nuage.lua

138

Tangentially, to silence luacheck, remove local, as f was already defined on line 92.