Make get_all_server_info and get_scaling_group_servers use pure-http

The metrics.py code had to be updated to deal with this, and I refactored the effectiness a little bit into metrics.py, so that there's a new get_all_metrics_effects function extracted from get_all_metrics.

I've added unit tests for get_all_metrics_effects, and left the unit tests for get_all_metrics in place (and updated them to ensure that code is using pure-HTTP properly). The tests for that are pretty nasty, but hopefully they'll go away if more of metrics.py gets Effect-ified.


effcloud isn't a thing. the original comment by manish read "txcloud", but since I got all Twisted dependencies in that function, I half-jokingly changed it to "effcloud".

The main blocker for actually working on effcloud would be to also finish the third-party HTTP client library that I started in the "effreq" github project (name up for debate). And I feel like I still need to do more bike-shedding on the API design of pure_http before releasing it to the public :P