Automatic merge from submit-queue

AWS: Allow cross-region image pulling with ECR

Fixes #23298
Definitely should be in the release notes; should maybe get merged in 1.2 along with #23594 after some soaking. Documentation changes to follow.

cc @justinsb @erictune @rata @miguelfrde

This is step two. We now create long-lived, lazy ECR providers in all regions.
When first used, they will create the actual ECR providers doing the work
behind the scenes, namely talking to ECR in the region where the image lives,
rather than the one our instance is running in.

Also:
- moved the list of AWS regions out of the AWS cloudprovider and into the
credentialprovider, then exported it from there.
- improved logging

Behold, running in us-east-1:

```
aws_credentials.go:127] Creating ecrProvider for us-west-2
aws_credentials.go:63] AWS request: ecr:GetAuthorizationToken in us-west-2
aws_credentials.go:217] Adding credentials for user AWS in us-west-2
Successfully pulled image "123456789012.dkr.ecr.us-west-2.amazonaws.com/test:latest"
```

*"One small step for a pod, one giant leap for Kube-kind."*

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