Add missing gov cloud region us-gov-east-1 for EC2
Currently, the AWS Gov Cloud feature in Foreman supports only a single region "us-gov-west-1" whereas, AWS supports two Gov Cloud regions.
Since it was just one region in the beginning, it was hardcoded inside the model. But, now it has two and this may increase in due course of time. So, it would be better to maintain multiple gov cloud regions and just add a region to it whenever support for a new one is made available.