Substitution mapping and selectable nodes

[lauwers]

I'd like to add support for mapping requirements to selectable nodes. I have a common use case where a host requirement in a substituted node needs to be mapped multiple times (to each of the host requirements of the nodes in the substituting template). Rather than defining multiple mappings, it would be more elegant to define a selectable host node in the substituting template and map the host requirement once to the selectable node. I think this could be done elegantly using a simple extension to the requirement mapping syntax as follows:

service_template:
  substitution_mappings:
    requirements:
      - server: [client, server] # traditional requirement mapping
      - host: compute # mapping to selectable node
  node_templates:
    client:
      type: Client
      requirements:
        - server:
            count: 1
       - host: host
    compute:
      type: Compute
      directives: [select]