[COOK-3557] Fix PubkeyAuthentication option

https://tickets.opscode.com/browse/COOK-3557

By using pub_key_authentication as the key, the option will be transformed into PubKeyAuthentication when the option is in fact PubkeyAuthentication http://www.openbsd.org/cgi-bin/man.cgi?query=sshd_config&sektion=5