support traffic split with shadow traffic

[szuecs]

This is not trivial and we might think about how to do this. I guess we should have 1st class support in fabric for this kind.
In case you want to do shadow traffic with a fraction of traffic, you need to create multiple routes as you can see in [1].

The current way fabric works is that we can not add these routes and a common solution is to add a RouteGroup in front of fabric, which doubles the proxy requests:

client -> skipper[RouteGroup] -> skipper[fabric] -> backends

Ideal case would be to be able to drop the RouteGroup, because of complexity.
I would expect, that people deploy a second application (likely with also fabric) and add in their old fabric configuration the new endpoint and a number between [0..1]. Numbers in the interval (0..1) are created in the way [1] is shown. 0 means we do not apply any new route, 1 means we do not to split the routes (tee() is enough).

[1] https://opensource.zalando.com/skipper/tutorials/shadow-traffic/