Add 'required' attribute to OS::Heat::CWLiteAlarm properties

Add 'required=True' relying on 
http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-cw-alarm.html
 
Change-Id: I524729c084db9976fcefca8d7ebc16abd0c92ffe
Closes-Bug: #1395711
Merge "Add 'required' attribute to OS::Heat::CWLiteAlarm properties"