network: add metering api operations

This patch aims to bring new tests related to Neutron Metering plugin.

They cover:
- List / Create / Show / Delete metering labels
- List / Create / Show / Delete metering label rules

Note: Update operation is not covered by the API.
Also add basic operations in network admin client.

Change-Id: Ieaf9a5dcd52f54e015e7565f50a3ea81569c483a
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
Merge "network: add metering api operations"