Remove optional notifier keys from glance

In the glance chapter of the installation guide, we configure
the notifier to use messaging. Glance notifications only appear
useful to environments using the glance ceilometer agent. Not
all environments use ceilometer and the associated queue could
become large without a consumer. We can remove the notifier keys
from the glance chapter since the glance ceilometer agent section
includes them.

I will fix this issue in the Juno version with a separate patch.

Change-Id: Id4e4dd97494dc84cf76203d54a92e035ffd997f6
Partial-Bug: #1360552
Merge "Remove optional notifier keys from glance" into stable/icehouse