Provide option to allow password server login

[colin-nolan]

Currently, password based authentication is disabled with no setting to enable it:
https://github.com/dev-sec/ansible-ssh-hardening/blob/master/templates/opensshd.conf.j2#L133-L134

It would be nice if there was a ssh_server_password_login setting (which could default to false).

[colin-nolan]

WIP.

[rndmh3ro]

Should be in the style like the other yes/no variables, see for example: https://github.com/dev-sec/ansible-ssh-hardening/blob/master/templates/opensshd.conf.j2#L131

UsePAM {{ 'yes' if ssh_use_pam else 'no' }}