Add Ganglia Pollster

* bascicly forward ganglia metrics to Ceilometer API
* mertrics fetched from Ganglia are configurable
* the resource of Posted Samples is configurable :
 - by default use the hostname of ganglia endpoint
 - can be forced by conf : all samples will attached with that resouce
 - use a ganglia metric (string) to become the resource.

Add HTTP publisher
 * the loop is closed, we can now to cascade Ceilometer.
 * Note: obviously samples POSTed are not signed

TODO:
* unittest
* doc

Part-of-Blueprint : applications-metrics-from-instances

Change-Id: Ief967a732f85748f0207b56788863d46718fb9fe