Add aws to allowed values on cluster autoscaler cloud-provider flag

[crigor]

This change is

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[k8s-github-robot]

[CLA-PING] @crigor

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[crigor]

I signed it!

[mwielgus]

LGTM

[mwielgus]

@crigor Apparently you need to sign one more CLA https://cla.developers.google.com/.

[crigor]

@mwielgus I've now signed that too

[googlebot]

CLAs look good, thanks!

[k8s-github-robot]

Automatic merge from submit-queue