Tolerate missing detached EC2 instances

[hwoarang]

Sometimes we see the following error during a rolling update:

I1125 18:12:46.467059 165 instancegroups.go:340] Draining the node: "ip-X-X-X-X.X.compute.internal".
I1125 18:12:46.473365 165 instancegroups.go:359] deleting node "ip-X-X-X-X.X.compute.internal" from kubernetes
I1125 18:12:46.476756 165 instancegroups.go:486] Stopping instance "i-XXXXXXXX", node "ip-X-X-X-X.X.compute.internal", in group "X" (this may take a while).
E1125 18:12:46.523269 165 instancegroups.go:367] error deleting instance "i-XXXXXXXX", node "ip-X-X- X-X.X.compute.internal": error deleting instance "i-XXXXXXXX", node "ip-X-X-X-X.X.compute.internal": error deleting instance "i-XXXXXXXX": InvalidInstanceID.NotFound: The instance ID 'i-XXXXXXXXX' does not exist
status code: 400, request id: 91238c21-1caf-41eb-91d7-534d4ca67ed0

It's possible that the EC2 instance to have disappeared by the time it
was detached (it may have been a spot instance for example)

In any case, we can't do much when we do not find an instance id, and
throwing this error during the update is not very user friendly.

As such, we can simply report and tolerate this problem instead of
exiting with non-zero code. This is similar to how we handle missing
EC2 when updating an IG[1]

[1] #594

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[k8s-ci-robot]

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[hakman]

Thanks @hwoarang!
/ok-to-test
/cc @olemarkus @johngmyers

[olemarkus]

Hey @hwoarang! So we contribute to the same project again :) Thanks

/lgtm

[hwoarang]

@olemarkus it's a small world :)

[hakman]

/approve

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