Use constructor for ecrProvider
Rename package to "credentials" like golint requests
Don't wrap the lazy provider with a caching provider
Add immedita compile-time interface conformance checks for the interfaces
Added comments
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."*
