Change list policies format in model (ready for review)

Apologies for the many little tiny changes.

I don't really remember why the policies were returned as a dictionary, since the API turns it back into a list anyway, except that maybe it started out being a dictionary in the mock model.

In any case, it doesn't make much sense particularly in the case of pagination.  The API needs to turn it into a sorted list, so that the last element can be used as a marker.

If this were not changed, in the case of cassandra, we'd fetch a correctly sorted list of policies, pull out the ids and put it into a dictionary, then change back into a list while stuffing the ids back into the policy dictionaries, and then re-sort the list into the order it came out of cassandra in anyway, which is kind of wasteful.

This does not make any difference in the behavior of the system, just removes extra transformations (which will be useful for paginating policies)