Add Ability to populate parameter dropdowns from another source ( PDB, etc.. )

The ability for the drop downs to be dynamic based on a PQL query.  This will help make plans more use friendly, and in my case here, provide the ability to select a business application and an environment, and the plan will go off and query facts to select the appropriate servers to deploy the task on.  

See comments on ENTERPRISE-1357 for more discussion around allowing parameter validation against a PQL query.