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