Fix instance_usage_audit_log test to use admin context

Nova V2 API 'os-instance_usage_audit_log' is for admin only
and Nova policy.json has rule for the same But
test_instance_usage_audit_log tests this API as non admin.

This patch fix test_instance_usage_audit_log test to use admin
context.

Also adding some more tests to verify this API with non-admin context.

Closes-Bug: #1387901

Change-Id: I20a477786ef0f3e1132629c4a12af40524e2b09f
Merge "Fix instance_usage_audit_log test to use admin context"