Add sftp-only support to openssh

Summary:
Configures sshd_config with the appropriate sftp-only subsystem based on
the unix group `sftp-only`.

References:

 * https://en.wikibooks.org/wiki/OpenSSH/Cookbook/SFTP
 * https://tickets.opscode.com/browse/COOK-2073
 * https://tickets.opscode.com/browse/COOK-2870