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.
I agree on storing full `lb_config` in DB since `delete_server` needs to know whether to remove from CLB and RCv3. Please note that there can be both CLB and RCV3 in the configs. 

I think it will be easier to have separate `remove_from_rcv3` function instead of reusing `remove_from_load_balancer`. This is because rc3 has bulk delete API that needs only server ids. You can have a simpler function that has one POST call for all RCv3 LBs. Also it uses cloud server ID instead of node_id in its docs.