first cut at -uc --usecloud ip:port at command line and usecloud= param in build_cloud()
(command line always dominates)
not sure if working yet, but tested that no breakage to existing behaviors
(normal build_cloud(), build_cloud_with_hosts(), command line -cj <cloud json>, command line -ccj <clone cloud json>,  for instance, still work)
I guess in a test, build_cloud() starts to behave more like a superset of h2o.init()