Enable Neutron metering extension by default

Neutron metering extension should be enabled by default because
it's required for some Tempest test. Also there's no need to set this option
for compute nodes

Change-Id: I401b51895c7dd3ba8bd8734640f5afa72c3d77dd
Closes-bug: #1387658
Merge "Enable Neutron metering extension by default"