add_to_load_balancer -> add_to_clb

This builds on top of #738, and that PR should be reviewed first.

This function was really always CLB-specific. This PR makes that clearer. Because the logic is so CLB-specific, I think it is an appropriate level to implement the alternative, RCv3 code path at.

I have cleaned up some of the tests and docstrings along the way. This code touches a bunch of tests that I quite frankly don't trust, so I think a full set of functional tests should be run.
Not really blocking - might be nice, but as this PR is pretty high priority, and this comment is also a temporary condition until the adding to RCv3 code path is implemented...

- Would it make sense for there to be a test that non-CloudLoadBalancer types be ignored?  
- Also, if they are ignored, the return value of this function is `None`, which in turn gets zipped up with the load balancer configuration in `add_to_load_balancers`, and I am not sure that `delete_server` knows what to do with a `None` value for `lb_response`.  Would it make sense to either filter out None values here or handle them in `delete_server`?

This *may* be useful later to handle non-CLB and non-RCv3 types, but then again the api should prevent non-CLB and non-RCv3 types from being passed in in the first place.