New API (helper) to retrieve Kill Bill region

[pierre]

Kill Bill should export the region it's deployed in (colo, us-east-1, etc.).

Implementation could be as easy as expecting a system property org.killbill.server.region (which we could also expose in KillbillServerConfig) and retrieving it from OSGIConfigProperties.

Plugins could then use it for region-specific properties, e.g. the Adyen plugin could look for org.killbill.billing.plugin.adyen.{region}.paymentUrl on top of org.killbill.billing.plugin.adyen.paymentUrl to see if a region-specific URL is defined.