Heat/flavor input constraint

Allow for template authors to declare a constraint on an input/Parameter that restricts input to match an available nova flavor.

Example

inputs:
  flavor:
    description: WebServer instance type
    type: String
    default: "m1.large"
    # constraints validate input values prior to provisioning
    constraints:
    - type: nova.FlavorChoice
      description: must be a valid instance type

Depends on Parameter Constraints