Use generic lb models from #872

This is based off of #872 - this actually removes the current `LBNode` and `LBConfig` and uses `CLBNode` and `CLBDescription` in the current code.

Future work (other PRs) left to do after this:  changing the converge logic to operate on generic `ILBNode`s and `ILBDescription`s.

Review question: wondering if we should change AddNodeToCLB to just take dicts of useful information, so if we change the underlying model again we don't have to change it to take that model instead (luckily in this case `LBConfig` and `CLBDescription` are almost the same).

These are the actual changes between #872 and this:  https://github.com/rackerlabs/otter/compare/generic_lb_models...use_generic_lb_models