PoC : Ganglia Pollster

Contains a new pollster (with no RPC) connection which poll Ganglia metrics
and publish metrics to Ceilometer Rest API.

TODO : unittests and further remarks

Change-Id: Ic548a714af228ab7808e4e62fc0e8712b33e36ed
use special metrics to add extra data to collected published to
ceilometer.
fix bug: extra metrics mapping per cluster/host
use socket to fetch XML from gmetad/gmond
fixe socket option missing
Don't publish metrics not consolidated by 'special' metrics

It's occur when the 'stack' gmond module is not deployed on a host.
fix typo
missing import
rm comments
rename agent : s/metric/satellite

+ fixes convertion metric type support
use oslo.config to set metrics mapping configuration