Add Neutron Metering Agent support

In Havana, Neutron has now a Metering Agent which gets meters from
virtual routers.

This patchs aims to allow devstack using this new service.

Change-Id: I17ad83799d60384247b98cc8a93ac032f641c721
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
Merge "Add Neutron Metering Agent support"