Fix template contributions retrieval on explorer

I thought template contributions placed in the nxserver/config file were not processed by the explorer, but they are (only the ones in templates that are activated).

Their process is a bit confusing though, as they appear as being contributed from the nuxeo-runtime bundle (because that's the one doing the introspection). Corresponding code in OSGiRuntimeService#loadConfigurationFromProvider holds a TODO "use a OSGi service for this.", maybe we can find a better way to introspect these.

See sample on the default cluster-config.xml contribution: http://localhost:8080/nuxeo/site/distribution/server-11.1-SNAPSHOT/viewContribution/cluster-config--configuration:

Also, the explorer defaults on bundle group "org.nuxeo.misc" for bundles who do not reference any bundle group --> see why the following are currently selected.

• org.nuxeo.osgi.app
• org.nuxeo.ecm.config
  see https://explorer.nuxeo.com/nuxeo/site/distribution/Nuxeo%20Platform%20LTS%202019-10.10/viewBundleGroup/grp:org.nuxeo.misc