Add a gnocchi dispatcher for ceilometer

This introduces a new dispatcher for Gnocchi that is able to
transform and publish the Ceilometer samples to Gnocchi.

Co-Authored-by: Julien Danjou <julien@danjou.info>

Change-Id: I424b991075ddc6ca7ef6bd7c6d17db247bf90c83
Merge "Add a gnocchi dispatcher for ceilometer"