Add some docs on how to setup Ceilometer to use Alarms

We need to use Ceilometer's mongo backend if we want to use
alarms as the sql backend is missing a feature.

Works around bug #1223710

Change-Id: Ib569cfd650ce14480496530e7bbf1b2f71a24c02
Merge "Add some docs on how to setup Ceilometer to use Alarms" into milestone-proposed