support ncm-metaconfig based on TextRender 

Additional fixes and unittests to allow ncm-metaconfig based on CAF::TextRender 
I don't like CAF making the decision of failing the entire component. This must return an error code (something false), but not call `error` or the component won't be allowed to recover.

It's a wrong design decision in ncm-ncd, but we can't fix it for 14.10.