Draining attributes in structures

This is so that @cyli can work independently on implementing the algorithm while I work on filling these structures.
Wondering if we should add `exclude_from_cmp=True` to this, since it will be 0 on all the LBNode configs, and is something not really intrinsic to a node anyway.  I realize this isn't a great place to put draining_timeout conceptually.

I was thinking of doing the same thing as this PR, actually :)  And was going to add the following caveat:

    TODO: this is not the ideal place for ``draining_timeout``, since
    :class:`LBConfig` is more about what node on a CLB should look like.
    And :class:`LBConfig` may not be applicable to a hardware LB.  It is also
    possible that ``draining_timeout`` does not apply to hardware LBs.

    So `draining_timeout` is here until other load balancer requirements are
    understood and a better place is found for it.