Convert server_usage v3 plugin to v2.1 API

Changes required to have v3 plugin natively support the V2.1
API.

The parameters is changed from os-server-usage to OS-SRV-USG

Partially implements blueprint v2-on-v3-api
Co-Authored-By: Chris Yeoh <cyeoh@au1.ibm.com>

Change-Id: I4a4ebe533f8910a29b54d6ec9554cc151af492d1
Merge "Convert server_usage v3 plugin to v2.1 API"