Enable Neutron metering service plugin

Aims to enable Metering plugin when using Neutron in the gate. It will allow
us to run Tempest tests against devstack-gate.

Change-Id: Id61aa17f2d66a5a56e8421dc63c3017d75198461
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
Merge "Enable Neutron metering service plugin"