[E2E] Command.Execute: return stdout and stderr separately

[barkbay]

This is a quick and dirty hack to workaround #5668.
Definitely not an ideal solution, happy to close it without merging.

[pebrc]

I wonder if we could simply distinguish between stdout and stderr to work around this issue. It looks like we are using CombinedOutput which mixes both and makes it hard to dismiss stderr for parsing.

cloud-on-k8s/test/e2e/test/command/command.go

Line 29 in 20f522f

return cmd.CombinedOutput()

But the deprecation and warning messages are output to stderr in AWS CLI (which acts here as authentication plugin to kubectl I believe) here:

https://github.com/aws/aws-cli/blob/d0ab1c4864bb015349bf0f8bd34329855c0bf116/awscli/customizations/eks/get_token.py#L177

And the server/client difference warning in kubectl also goes to stderr here:

https://github.com/kubernetes/kubernetes/blob/ea0764452222146c47ec826977f49d7001b0ea8c/staging/src/k8s.io/kubectl/pkg/cmd/version/version.go#L162