Enhance API parameters validation when create Bay

Currently the Magnum API parameters validation is not enough to prevent further API failure, due to non-existed or unprivileged OpenStack resources can be referenced and there're no check/validation toward them. We should check whether the OpenStack resources used by Magnum is existed when create or update Magnum resources. For example, when we create a Baymodel, we should check the existence of image, flavor, network in API parameters. If we didn't do this and Baymodel with non-existed image_id is successfully created, the Bay creation use this Baymodel will fail.