Add stdin input for secrets

[ihoegen]

This PR allows for stdin input for creating kops secrets.

Now where a secret can be created via file, it can be created via stdin.

Example:

encryption-config-generator.sh | kops create encryptionconfig -f -

[chrisz100]

Good stuff, thanks!
I’d say wait for 1.12 though as it’s touching base components that could break our stable release?
/lgtm
/assign @justinsb

[justinsb]

We've cut the 1.11 release branch, so we can cherry-pick selectively if we want to. But I don't think it's that risky - but maybe I'll live to regret that!

Thanks @ihoegen

/approve

[k8s-ci-robot]

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This pull-request has been approved by: chrisz100, ihoegen, justinsb

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