Create new model policy framework and extend it to services

We need to take a look at how model policies are implemented. These are both model policies proper (the OpenStack synchronizer's model_policies directory) as well as ad hoc policies that exist in django model code (the stuff Sapan had to split out when running the generative toolchain). Of the "gap" items listed in this message, this is probably the most open ended one, as I think we don't entirely know how to do it yet. We have a desire to make it as declarative as practical, yet at the same time we have a diverse set of policies that suggest we need fair amount of flexibility.