backend server weight support

[noahdietz]

We should add support for weighting the backend servers (which are Pod replicas in this case) with native nginx server weights

This has come up in previous conversations, but never tracked.

I bring this up because I want us to implement a canary and/or blue/green testing feature using this approach. Through the addition of some label, a user can indicate that a dev version of a deployment should take over some percentage of traffic in production (handled with nginx weights), until the dev deployment is removed, or if the dev deployment becomes the production deployment through a slow roll over of traffic.

Thoughts? (this would be something for GA or GA-2, considering this is a nice-to-have)