artifact definitions need clarification

[lauwers]

Tal's artifact definition example shows an artifact definition in a node template the refines an artifact definition in the corresponding node type. Neither the TOSCA v2.0 spec nor the TOSCA Simple Profile in YAML v1.3 spec suggests that this is allowed. In fact, the artifact definition in the TOSCA v2.0 spec states that if an artifact is redefined, no keyname definitions are inherited, which suggests that "refinement" is not allowed.

By the way, it has always bothered me that artifacts are handled different than all other entities in TOSCA. All other entities are defined in types definitions and assigned in templates. We should consider introducing separate artifact definitions in node types and artifact assignments in node templates.