Use service account token instead of apiserver basic auth

[bergerx]

It'd be better to use a service account token rather than basic auth here:
https://github.com/gardener/vpn/blob/4fdadc60390d275a02a8289ca2e58d0a2c9ac7c4/seed/example/seed-deployment.yaml#L44

Some historic discussion could be found here:
https://kubernetes.slack.com/archives/CB57N0BFG/p1532940549000137

[petersutter]

related slack thread: https://kubernetes.slack.com/archives/CB57N0BFG/p1532940549000137