Heat/glance image input constraint

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

Example

inputs:
  image:
    description: WebServer image
    type: String
    default: F18-x86_64-cfntools
    # constraints validate input values prior to provisioning
    constraints:
    # validates that the value matches one of the available
    # glance images
    - type: glance.ImageChoice
      description: must be a valid image name

Depends on Parameter Constraints