Flexible use of Requirements on Node Templates

There was a notion that was discussed during TOSCA-117:
see: https://tools.oasis-open.org/issues/browse/TOSCA-117

That added the the concept of allowing more flexibility in matching applications to infrastructures and not dependent on strict node type requirements matching. TOSCA-117 introduced a capability/requirement constraint proposal to allow constraint expressions such as: "greater than", "one of", etc.

This JIRA issue intends to discuss another possible mechanism to allow Node Templates the ability to have OPTIONAL or DESIRED requirements that do not necessary HAVE to be matched in order for the application to deploy/run, but provide an orchestration engine additional requirements that if available (i.e. exported as a capability only in some environments, or infrastructures) could be used for a "better" match.

To add this concept, the notion of "fit for purpose" workloads is a term I (Matt RutkowskI) am using frequently in talking about TOSCA. This means that some apps or "workloads" might be designed to work "better" on certain infrastructure, that TOSCA should be able to communicate in its application patterns (node templates) the not only the types of IaaS (infrastructures) it COULD run on, but also optionally in the same Node Templates provide "hints" (using TOSCA's requirements construct) to tell the engine where the workload could run "better" or even "best" (ideal match).

To this end, Derek kindly prepared a document describing an early proposal working in general agreement from Travis and Kevin (HP) on the concept, although the details still need review and discussion at both the SC and TC (see "Proposal section" below)