Camel-scr todos and polish

Visibility / access:

• Make context private. Use getter.
• Get rid of registry member variable. Get registry through context when needed.
• Make methods that are not supposed to be overriden final.

Customization:

• User can customize context's registry by overriding createRegistry() methods. Returns SimpleRegistry by default.
• Remove hardcoded defaults from setupCamelContext().

Corrections:

• Do not explicitly create an OsgiServiceRegistry for OsgiDefaultCamelContext. The context will create one anyway and combine it with whatever createRegistry(BundleContext) returns in a CompositeRegistry.
• Make camel-archetype-scr integration test run with Maven 3.3.9.