Extract the activemq fabric discovery into a fragment

The fact that the fabric discovery is not in a fragment can lead to issues if the activemq bundle is started after the client.  This is because the activemq bundle has an activator which scans other bundles for service extensions, and in particular for discovery implementations.  Thus, the activemq bundle needs to be started before any other bundles that actually use activemq.  However, OSGi has no way to force such a thing and using start levels can only limit the chances of hitting the problem, as any kind of refresh/restart of bundles may not take the start levels into account.  
Extracting the discovery into a fragment makes sure that the discovery protocols are available when activemq is used, wether the bundle is started or not.
In order to avoid the split package, the package has also be renamed to io.fabric8.mq.fabric.discovery.