Add support for traffic segments.

[gargravarr]

This Pull Request (PR) adds support for traffic management through the use of
Skipper's TrafficSegment predicate. By default this feature is not active for
StackSets, and can be activated/controlled in the following way:

• If a new StackSet includes the annotation "stackset-controller.zalando.org/use-traffic-segments": "true", the controller will manage ingress/routegroup and traffic with a dedicated resource per stack (a Ingress/RouteGroup segment), using TrafficSegment to split traffic. This annotation is only active when you start the controller with both --enable-traffic-segments and --annotated-traffic-segments set.
• If someone (or some entity) updates an existing StackSet with the annotation
  "stackset-controller.zalando.org/use-traffic-segments": "true", the controller
  will convert the StackSet to use TrafficSegment in the following way:
  1. Creates Ingress/RouteGroup segments for each Stack, with the segment
     corresponding to each Stack traffic weight.
  2. Deletes the central Ingress/RouteGroup, only after the controller created
     per Stack segments.
• Starting the controller with the option --enable-traffic-segments enables by default traffic segments on all StackSets. In this mode the controller will also convert StackSets using a central ingress resource as described above.
• Starting the controller with the option --enable-traffic-segments and --annotated-traffic-segments enables traffic segments only on StzasckSets with the annotation "stackset-controller.zalando.org/use-traffic-segments": "true".

On StackSets with traffic segments, the controller switches traffic one segment
at a time, prioritizing first stacks with a higher increase in traffic share.

This PR also updates end 2 end tests to repeat the same tests also with StackSets with explicit support for Traffic Segments.

Finally, this PR removes deprecated varcheck and deadcode linters,
already covered by unused.

[mikkeloscar]

👍

[gargravarr]

👍