Fix cassandra vault stats, use dicts instead of tuples. 

This patch does a couple of things:

1) Fixes cassandra driver so it can run against a real cassandra instance. This was a regression caused by over overly liberal mocking framework.
2) Adds the ability to use dictionary-based arguments in our mocker.
3) Changes all arguments to be dictionary-based. 

Closes: CBDEUCE-154
Closes: CBDEUCE-141.


Q: How about we replace this with "assigned block count" and "unassigned block count" fields (perhaps in another PR)?

This could be stored as res['blocks']['assigned'] and res['blocks']['unassigned']. res['blocks']['count'] should equal their sum.