ceph: add support for auth_x authentication protocol

The auth_x protocol implements support for a kerberos-like mutual
authentication infrastructure used by Ceph.  We do not simply use vanilla
kerberos because of scalability and performance issues when dealing with
a large cluster of nodes providing a single logical service.

Auth_x provides mutual authentication of client and server and protects
against replay and man in the middle attacks.  It does not encrypt
the full session over the wire, however, so data payload may still be
snooped.

Signed-off-by: Yehuda Sadeh <yehuda@hq.newdream.net>
Signed-off-by: Sage Weil <sage@newdream.net>