Added new methods startScan(bindingId), stopScan(bindingId), supportsDiscovery(bindingId) and supportsDiscovery(thingType) to DiscoveryServiceRegistry.
Added support for multiple discovery services for the same type.
Fixed not stopping scheduler, when start fails in AbstractDiscoveryService.
Adapted tests.

Signed-off-by: Dennis Nobel <d.nobel@external.telekom.de>