Add sshd_config file validation in lineinfile task

[Jeff Geerling]

See: https://www.youtube.com/watch?v=gV_16dU7XjM

And see related issue from my book, Ansible for DevOps: geerlingguy/ansible-for-devops#273

[Jeff Geerling]

The line to add: validate: 'sshd -t -f %s' (see note on -T vs -t in man docs, as mentioned by @KaliszAd).