The SDC janusgraph DB must be updated to support the definition of models

A new vertex type called "model" is to be added to represent models

A new edge type called MODEL_ELEMENT is to be created to connect a tosca type to a model

A new edge type called MODEL is to be created to connect a resource/service to the model it is based on

See attached image for how the model is created in the DB. Note a lot of the relationships for the service and other types (capabilities etc) omitted for simplicity. In the diagram, the colour green indicates what we need to add

REST endpoint to be provided from defining the model. For now need only take the model name (more will be added in later stories e.g. the import file)