Support dark release deployment

[otrosien]

An interesting feature instead of starting with canary, and directly shifting traffic to your new stack, it would be nice to first push some shadow traffic to the new stack, and then decide to switch real traffic.

https://www.functionize.com/blog/what-is-canary-testing-and-dark-launching/

[szuecs]

This could be done with the RouteGroup support in stackset-controller starting with 0% traffic switch and shadow traffic similar to https://opensource.zalando.com/skipper/tutorials/shadow-traffic/
@otrosien the question would be how the user interface should look like in case we need to do something more in stackset controller.