Disable POST samples by non-admin users to meters

Enabling users with non-admin privilege to execute this method can potentially
cause "denial of service" attack, a serious security vulnerability.

Also added unit test code for this change.

Background:

The current implementation of Ceilometer allows non-admin users to add
user-defined metrics to be reported on by Ceilometer. There is currently
no mechanism to limit/rate such operation making it easier for a rogue
user to launch denial of service attacks. Limiting operation to admin
users only mitigates this thread.

Not-in-upstream: true
Implements: rally-user-story US4145
Implements: rally-task TA3484