Added configuration parameter, send_cluster_metrics, to magnum.conf
with default value of True. If set to True, periodic tasks will pull
COE data and send to ceilometer. This parameter can be set to False to
disable periodic collection of data to avoid unnecessary load from the
cluster.
Closes-Bug: #1668330
Related-Bug: #1746510
Change-Id: I9945293e7b2b52731f6e220d0925c1f6ad097caa