Neutron Metering Agent has been introduced in Havana release.
This patch adds the agent in Neutron Diagram and explains the process to
install the agent on the network nodes.
Change-Id: Id8f16aab44af18141a63fa873d9f856f83f444a4
backport: havana
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>