add tests for InstanceUsageAuditLog

this adds both positive and negative tests for the
following client:
GET /v2/{tenant_id}/os-instance_usage_audit_log/%s
GET /V2/{tenant_id}/os-instance_usage_audit_log

also adds corresponding xml and json rest client

Change-Id: I4991c433c35248558dbd655882a3cb30a88a9f15
Merge "add tests for InstanceUsageAuditLog"