Add support for other ingress controller

[arjunrn]

Current the controller is tied to Skipper as an ingress controller and the dependency is for the traffic switching annotation. The controller should support other ingress providers as well such like Traefik which works exactly the same as skipper with the annotation traefik.ingress.kubernetes.io/service-weights. Nginx behaves a little differently because it doesn't define the multiple backends on the same ingress but allows multiple ingresses with the same host.

[szuecs]

Traefik already agreed on using ours, see https://twitter.com/timoreimann/status/1107792769421111296

[szuecs]

I think with https://github.com/zalando-incubator/stackset-controller/releases/tag/v1.1.0 we enable anyone to create whatever resource to build the LB infrastructure.

[szuecs]

I thought about it again and realized, that we could pass the annotation name via flag, default to zalando.org/backend-weights, which should fix the issue much better.