Export HealthCheckRegistry as an OSGI service

[pierre]

In KillbillActivator, register the HealthCheckRegistry as an OSGI service, so that plugins can access other plugins and Kill Bill healthchecks.

killbill-platform/osgi/src/main/java/org/killbill/billing/osgi/KillbillActivator.java

Line 205 in 4fa797e

registrar.registerService(context, MetricRegistry.class, metricsRegistry, props);

[reshmabidikar]

Addressed in #180. Closing this ticket.