Merge pull request #24464 from matthewrudy/aws-ap-northeast2

Automatic merge from submit-queue

AWS: More support for ap-northeast-2 region

Issue #24446

The new AWS region for Seoul, Korea (ap-northeast-2)
was launched in January 2016
https://aws.amazon.com/blogs/aws/now-open-aws-asia-pacific-seoul-region/

But it requires a few changes.

To test:
```
export KUBERNETES_PROVIDER=aws
export KUBE_AWS_ZONE=ap-northeast-2a
export MASTER_SIZE=t2.medium
export NODE_SIZE=t2.medium
export NUM_NODES=4
cluster/kube-up.sh
```

I assigned the AMIs by checking the specific version used from `ap-northeast-1`,
and finding the same image with the same datestamp.

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cluster/aws/util.sh

@@ -365,6 +365,10 @@ function detect-trusty-image () {
         AWS_IMAGE=ami-93876e93
         ;;
 
+      ap-northeast-2)
+        AWS_IMAGE=ami-62ac620c
+        ;;
+
       ap-southeast-1)
         AWS_IMAGE=ami-66546234
         ;;