Calculation of weights for un-weighted backends by controller

[CarltonSemple]

Calculating the weights for un-weighted backends by the controller would offload some of the work from the sidecar. This weight calculation could potentially be returned in the controller API as implicit_weight

[rshriram]

I think we should avoid any more fields in the JSON schema, especially for things like this. Two options:

1. We could actually replicate Nginx behavior and avoid the % based weights totally. In other words, you can specify any positive weight for a backend (100, 200, etc.). The controller can distribute the computed weights accordingly across the backends.
2. We could stick with existing weights (0-1.0]. In this case as well, the controller computes the weights for unweighted backends before passing them on to the sidecar.

[frankbu]

I prefer the second option. Note that this will be a breaking change from the existing GET implementation, but maybe that's not so bad.

[rshriram]

I think this has been handled in the recent changes to controller. @ijsnellf @GregHanson ?