add regexp format validation to InstanceType param

This param now supports 1-4 instance types, provided they're separated
by commands. That's not as obvious as I'd like it to be, so this adds a
more descriptive error if we've failed to explain ourselves well enough
and a user gets the format wrong.

Author: yob

templates/aws-stack.yml

@@ -208,6 +208,8 @@ Parameters:
     Type: String
     Default: t3.large
     MinLength: 1
+    AllowedPattern: "^[^,]+(,[^,]*){0,3}$"
+    ConstraintDescription: "must contain 1-4 instance types separated by commas"
 
   MaxSize:
     Description: Maximum number of instances