Missing functionality for setting different aws partitions

[tielong]

Hello,

First of all thanks for creating this great sample! It is super helpful and saved us a lot of development efforts.

I noticed that currently the sample doesn't allow setting aws partition as an override parameter. I am based on China and most of our resources are located within aws-cn instead of aws.

I was able to manually update the template file as well as the lambda function to make our case work, but I could see how adding the ability to override partition could benefit future users, especially considering we have at least 3 partitions for AWS now: aws, aws-cn, aws-us-gov, and that list might grow in the future. (Similar to "TargeRoleName", "TargetRegion" maybe we can have "TargetPartition")

Thanks a lot and thanks again for this awesome sample!