Add metering agent support

The Ceilometer project is able to get meters from Neutron via the
notifications system on how much bandwidth the projects are using.
The idea is to meter this as the L3 routers levels.

The feature exists since Havana.

Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
implement blueprint metering-agent

Change-Id: I62ad9b7455e5f2bc3455b55a346f2515ca1040a3
(cherry picked from commit bf34f8631cfae0550dbe082740668df82f0cdfd4)