extend capabilities/GetProviders to provide the default provider

The app manager so far does not allow to specify a provider for an interface. Hence, always the default provider is used.

For applying the correct remappings I need to know the default provider for an interface.
My suggestion would be to add this information to the capabilities/GetProviders service.

E.g.:
```
string interface
---
string[] providers
string default_provider
```

Thoughts?