Automatically build RegionMap for common AMIs

[rcrowley]

Having to do this by hand is stupid:

  "Mappings" : {
    "RegionMap" : {
      "us-east-1" : {
          "AMI" : "ami-76f0061f"
      },
      "us-west-1" : {
          "AMI" : "ami-655a0a20"
      },
      "eu-west-1" : {
          "AMI" : "ami-7fd4e10b"
      },
      "ap-southeast-1" : {
          "AMI" : "ami-72621c20"
      },
      "ap-northeast-1" : {
          "AMI" : "ami-8e08a38f"
      }
    }
  },