Add msgpack as a dependency

Ceilometer will use msgpack for encoding data in, at least, its UDP
based transport.

msgpack is distributed under the Apache Software License. It seems to
support Python 3, and is already packaged for Debian, Ubuntu, Fedora,
and probably a lot more OSes.

Change-Id: I35ceb44046fe3faaa7aebf9a6e1f173acc7b1c02
Signed-off-by: Julien Danjou <julien@danjou.info>
Merge "Add msgpack as a dependency"