Error loading metering driver iptables

[yuqian]

Description
==============
When I running `neutron-metering-agent ...`, the program was interrupted。

Environment
===========
1.Version:
  neutron branch:master

Steps to reproduce
=====================
1.Install & Config
  Please refer to the official link for installation and configuration：
    https://docs.openstack.org/neutron/pike/admin/archives/adv-config.html
2.Execute
  Running `neutron-metering-agent --config-file NEUTRON_CONFIG_FILE \
    --config-file L3_METERING_CONFIG_FILE`

Logs
=======
2018-08-15 08:12:09.339 7 INFO neutron.services.metering.agents.metering_agent [[
-] Loading Metering driver iptables
2018-08-15 08:12:09.339 7 ERROR neutron.services.metering.drivers.utils [-] Erroo
r loading metering driver 'iptables'

Solution
==========
neutorn/setup.cfg:
  neutron.services.metering_drivers =
    noop = neutron.services.metering.drivers.noop.noop_driver:NoopMeteringDriver
    iptables = neutron.services.metering.iptables.iptables_driver:IptablesMeteringDriver
iptables‘s path should be: ’neutron.services.metering.*drivers*.iptables.iptables_driver:IptablesMeteringDriver‘