Use known_hosts ansible module instead of ssh-keyscan

[maximecolin]

http://docs.ansible.com/known_hosts_module.html

[boscowitch]

known_hosts module can't add remote keys therefore one would need 2 tasks acutally... one to get the remote key into a variable over stdout (probably also using ssh-keyscan) and then the known_hosts module to write it to the known hosts file ... would actually not be more performant but the upside might be no double adding of keys...