Generic LB model (interfaces) for converger.

Some generic LB model interfaces (and specific CLB interface providers).  
Here is a sketch of what planning will look like (not full changes, no tests):  https://github.com/rackerlabs/otter/commit/5d8637c9ed58bff73afce9b8b82feef60afe605c.

That particular commit has the adds and minuses reversed, since that commit is reverting the changes I made to sketch out what planning will look like.

There will also be specific RCv3 implementations of ILBDescription and ILBNode (the RCv3 version may not be drainable).

Also looking to implement `AddToLoadBalancing` etc. as a function which checks what the types are and produces the right specific steps.

Naming corrections/feedback about above welcome and requested :)