This pull request addresses the following:

1. The mesos collectd plugin now uses the preferred /metrics/snapshot
endpoint for gathering metrics.

2. The mesos collectd plugin now only reports metrics from the elected
mesos master.  This will result in less duplicate or meaningless metrics
being reported by non-elected mesos masters.
Updated the plugin description.
Merge pull request #2 from rayrod2030/feature/use-new-mesos-metrics-endpoint

Have only elected master report metrics from /metrics/snapshot endpoint