cubicweb-container #2551746 define_container: allow reuse of an existing rtype for container_rtype [resolved]

This will help plug container in existing applications where a fitting rtype is already there.

prioritynormal
typeenhancement
done in1.0.2
load0.100
load left0.000
closed by#38a8a7c83ac9 [utils/define container] replace the try/excepts with proper checks, also enable reusing an existing container_rtype (closes #2551746)