constraints should be validated

[Kapil Thangavelu]

Generic constraints map to an instance type, an unsolved instance type should result in an error at deploy/constraint set time. This problem only increases as we get to parity around constraints (instance-type, zone, etc). In all of these cases the provider vocabulary for the constraint is either introspectable or static in the code, and should be validated against the provider vocabulary when the constraint is set rather than resulting in a pending forever instance. In some cases constraints are dynamic so the async feedback mechanism (status collection) will need to be utilized for runtime provider vocabulary changes.