config: make System properties uniform

They all start with org.killbill.billing.plugin.analytics.

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
config: use OSGIConfigPropertiesService instead of System properties

When Kill Bill properties are loaded via a file, System properties
aren't set. The plugin should instead rely on OSGIConfigPropertiesService.

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>