This adds a mechanism for specifying user configurable cluster
parameters in a declarative way. The set of configurable cluster
parameters (as well as useful metadata like input type/bounds/valid
values, etc...) are exposed via a rest api. This allows creation of nice
interactive GUIs on the client side without requiring client-side
configuration (still TODO).

Still needs tests and nicer client-side interaction, but on initial
experimentation the server-side componts work.