Implement simple node template substitution

[aszs]

We want ensembles to have a public type, substitution_mapping can provide this.
There are a couple of simple versions of substitution_mapping to consider:

• just provider a node_template
• just provide a node_type only and treat topology_templates inputs and outputs as properties and attributes
  Should we add node_type to external declaration for validation?

Implementation notes:

• substitution happens in _handle_nested_tosca_templates_with_topology but only from imported templates, not from the "inventory"

• nodetemplate.sub_mapping_tosca_template is never used
• substitution_mappings is described as a list in TOSCA 1.3 grammar

• how to handle root instance? how to serialize/load?
• add an "export" directive? a "root" directive?
• loaded imported template with the node's properties and have the nodespec point to the substituted node_template

Alternatively

Instead of a substitution mapping what if we just added a directive named "export" or "root" or "public"?

  aNodeTemplate:
  type: mytype
    directives:
    - root
    properties: 
     foo: {get_input: foo}
    capabilities:
      kkk:
``