Added feature definition XML for core OpenNMS bundles to support adding
ActiveMQ sampler listener to the main OpenNMS system. Changed the name of
the core feature artifact to org.opennms.karaf/opennms to match the
convention that Camel and CXF use. Removed the org.opennms.core.* classes
from the Karaf system classpath and added opennms-core feature dependencies
to Vaadin apps that require it.