Fix luxi serialization in ganeti-masterd

Currently, lib/luxi.py used lib/serializer.py for encoding/decoding
messages, but the master daemon uses directly the simplejson module.
This is wrong as any non-trivial change to serializer.py will break the
master daemon.

The patch changes masterd to use exactly the same functions as luxi.py
for encoding/decoding of messages.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>