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.
To clarify: 

- The separate code paths don't actually exist yet.
- `remove_from_load_balancer` is CLB-specific, right now, but it will grow a separate code path for RCv3. In order to pick the code path, needs to know if the LB is RCv3 or CLB, and that information is in `lb_config`, but not in `lb_id`.