Update cloudwatch alarm spec

[csumpter]

• [x] Please ensure new code produces a valid CloudFormation template.
• [x] Tests pass.
• [x] Verified that new specs can produce a valid cloudformation template, and provision a resource to AWS.

Properties have been expanded since last update: https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-cw-alarm.html