Neutron: include metering service plugin in conf
Neutron: include metering service plugin in conf

The common code for metering calls _neutron_service_plugin_class_add,
which despite the description only just appends a service plugin to
$Q_SERVICE_PLUGIN_CLASSES - it doesn't actually write it into a
configuration file.

So for now, read out the configuration, and append metering to it, then
write it back out.

Change-Id: Ice96cca8b43dcd54f2aa81461000a4597db8260d

group-policy/devstack,openstack-dev/devstack,nawawi/openstack,noironetworks/devstack,nawawi/openstack,dtroyer/devstack,bswartz/devstack,noironetworks/devstack,openstack-dev/devstack,group-policy/devstack,richliu/devstack,bswartz/devstack,dtroyer/devstack,richliu/devstack