A per-tenant, per-region a LB cache

Ready for review - addresses issue #60
As per #70, this is a really bad name for this class and is just carried forward from the `lb_cache` naming convention. Unlike `S_Cache`, which is *itself* an `s_cache`-style dictionary, this is actually an object (albeit not one with a lot of interesting methods).  I think re-naming this class to something like `CloudLoadBalancerTenant` would be better.