neutron: Fix config snippet support for metering agent

[vuntz]

Because the metering agent is started with an explicit "--config-file /etc/neutron/metadata_agent.ini" flag, options in this file will override everything that is being set in config snippets.

So do not generate /etc/neutron/metadata_agent.ini (instead empty it), but instead a config snippet.

This wasn't done as part of 654c0ea1 because the neutron package was missing a /etc/neutron/neutron-metering-agent.conf.d directory back then.