Prepare for encryption support (first part). Add decryption and encryption key generation. Thanks to Metze for helping with this.

BugLink: http://bugs.launchpad.net/bugs/1670508Reviewed-by: Stefan Metzmacher <metze@samba.org>
Signed-off-by: Steve French <steve.french@primarydata.com>
(backported from commit 373512ec)
Signed-off-by: Joseph Salisbury <joseph.salisbury@canonical.com>