Allow resource search by physical_resource_id

Since heat allows to find resource by stack id, would be very usefull to find resource by physical_resource_id.

Suppose we have instance as part of stack provisioned by heat. And we want to perform several actions over given resource. Project which got stack provisioned stores in persisted models only compute server id, that is actually physica_resource_id of AWS::Instance.
Since project persisted models are storing only physical resource ids there's no ability to perform stack_update, resource_update actions on stack.

Heat should provide an ability to perform next quries:

Call: HTTP GET stacks/{physical_resource_id}
Response: HTTP 200, body: a list of resources which have equivalent physical resource id, if there are no resources - return empty body.