Update aws-matlab-template.json to support GovCloud

[bollig]

Added use of AWS::Partition pseudo parameter (https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/pseudo-parameter-reference.html#cfn-pseudo-param-partition). This enables template to function across partitions like GovCloud and China.

Users need to override AMI ID with their own built for us-gov-west-1, us-gov-east-1, etc.