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.