skipper: downgrade webhook image version

Use version before #6341.
PR #6384 introduces config item for ingress validation.

Signed-off-by: Alexander Yastrebov <alexander.yastrebov@zalando.de>

cluster/node-pools/master-default/userdata.yaml

@@ -243,7 +243,7 @@ write_files:
               name: admission-controller-kubeconfig
               readOnly: true
         - name: skipper-admission-webhook
-          image: 926694233939.dkr.ecr.eu-central-1.amazonaws.com/production_namespace/teapot/skipper:v0.17.1
+          image: 926694233939.dkr.ecr.eu-central-1.amazonaws.com/production_namespace/teapot/skipper:v0.16.167
           args:
             - webhook
             - --address=:9085