Add 'instance-usage-audit-log' plugin for V2.1

This patch adds 'instance-usage-audit-log' plugin for V2.1 API.
Also implement the API sample tests and share the unit tests
between V2 & V2.1.

Partially implements blueprint v2-on-v3-api

Change-Id: I152d00fb4b12942515d5db54a7ec381279c31ccd
Merge "Add 'instance-usage-audit-log' plugin for V2.1"