Do not enforce glance_control_exchange

The default glance_control_exchange in Ceilometer has been changed
from 'glance_notifications' to 'glance' in grizzly-2. Glance always
had 'glance' for its control_exchange.

This was a bug in Ceilometer and puppet-ceilometer patched it.
It shouldn't anymore.

Change-Id: I217515c91f092fb3e8039dde2d14a7ab4cbb7245
Closes-bug: #1273878
(cherry picked from commit 85d46894cca6ba80e8cb57243dde57e44cffa443)