The Monasca alerting pipeline provides multi-tenancy alerts and
notifications. It runs as an Apache Storm topology and generally
places a significant memory and CPU burden on monitoring hosts,
particularly when there are lot of metrics. This is fine if the
alerting service is in use, but sometimes it is not. For example
you may use Prometheus for monitoring the control plane, and
wish to offer tenants a monitoring service via Monasca without
alerting and notification functionality. In this case it makes
sense to disable this part of the Monasca pipeline and this patch
adds support for that.

If the service is ever re-enabled, all alerts and notifications
should spawn back automatically since they are persisted in the
central mysql database cluster.

Change-Id: I84aa04125c621712f805f41c8efbc92c8e156db9