Add option to also deregister the terminating instance from all ELBs

[piontec]

This becomes essential when instances are exposed with ELB: after getting the termination signal, the instance is drained and soon will be terminated. Still, kube-proxy keeps running on the instance, so it's not removed from ELB. WHen isntance stops, ELB must detect failure on its own, which introduces a delay of at least 10s. With this PR terminating instances can deregister themselfs from all ELBs before they will be reclaimed by AWS.

[pusher-ci]

@piontec: PR needs rebase.

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