ssh: rewrite ssh responder to use cache_req This is a bigger change since both supported commands could be rewritten for cache_req and the logic could be deleted. I decided to also split the file into more modules and follow similar pattern as with nss responder. Resolves: https://fedorahosted.org/sssd/ticket/1126 Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>