Update metering agent to use interface_driver alias

Currently the metering agent is using the old import method,
use stevedore instead.

https://review.openstack.org/#/c/419881/ merged long ago.

Change-Id: I4e5b8734f00cfa98cb60a70cc85b6c8924d9b718

templates/metering_agent.ini.j2

@@ -6,15 +6,7 @@ debug = {{ debug }}
 
 # Drivers
 driver = {{ neutron_driver_metering }}
-
-# Other agents will use neutron_plugins[neutron_plugin_type].driver_interface
-# for the interface_driver, but that uses a stevedore alias. Metering agent
-# hasn't been updated to use stevedore alises so that fails. To work around
-# the problem until metering agent is updated, we should use the full
-# module.class path to the interface driver.
-# TODO(hughsaunders): switch back to stevedore when
-# https://review.openstack.org/#/c/419881/ merges and is backported.
-interface_driver = {{ neutron_services['neutron-metering-agent'].interface_driver }}
+interface_driver = {{ neutron_plugins[neutron_plugin_type].driver_interface }}
 
 # Intervals
 measure_interval = 30

vars/main.yml

@@ -403,7 +403,6 @@ neutron_services:
     config_overrides: "{{ neutron_metering_agent_ini_overrides }}"
     config_type: "ini"
     init_config_overrides: "{{ neutron_metering_agent_init_overrides }}"
-    interface_driver: "{{ (neutron_plugin_type == 'ml2.lxb') | ternary('linuxbridge', 'openvswitch') }}"
     start_order: 3
   neutron-l3-agent:
     group: neutron_l3_agent