Improve sys::ssh:
 * Add parameters for using authorized keys commands and SSH CA certificates
 * Perform general cleanup on sshd_config template
Some further cleanup:
 * Actually add parameter for `trusted_user_ca_keys`.
 * Made default paramters for `ciphers` / `macs` be empty arrays.
 * Fixed bugs and other formatting tweaks to sshd_config.erb template.
Merge pull request #3 from counsyl/ssh-improvements

ssh improvements