Config help for Jetty instrumented does not show instrumentation properties

[andrus]

Running an app that has instrumentation on with -H does not display jetty.health property.

This is a general limitation of the Bootique core that does not recognize overridden factories as such (so the same problem will e.g. appear for InstrumentedHttpClientFactoryFactory in bq-jersey). We need to either redesign the factory subclass approach, or get back to the basics and use unique config prefixes for each module (`jettyinstrumented" in this case).