RegisterBusSubscribersCompilerPass: Add support for parameter return values

When using a parameter for my projector classes I received this exception:
```
ReflectionException: Class %my_parameter% does not exist
```
This PR solves this by asking for the parameter first before doing the reflection.

Edit: When looking if this was a known symfony bug http://stackoverflow.com/questions/19266204/after-tagging-a-service-in-services-yml-symfony-starts-throwing-reflectionexcept

Edit2: https://github.com/richsage/RMSPushNotificationsBundle/blob/master/DependencyInjection/Compiler/AddHandlerPass.php
@stof ok i'll remove no need for an extra test then