admin-guide-cloud: Document Neutron Metering Agent

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>
Merge "admin-guide-cloud: Document Neutron Metering Agent"