Increase StackSet Validation Frequency to 24 hours by default

With the new Config Pricing running the Evaluations every 3 hours
produces unnecessary costs where a 24 hour schedule (with parameter
to reduce that time) is sufficient.

main-account-stacks/05-validate-stack-set-deployments/validate_stack_set_deployments.template.yaml

@@ -3,6 +3,16 @@ Parameters:
     Type: String
     Description: Email address to notify in case the Validate Lambda function breaks
     Default: {{AWSMainAccount['Email']}}
+  ValidationFrequency:
+    Description: 'The frequency with which AWS Config delivers configuration snapshots'
+    Type: String
+    Default: TwentyFour_Hours
+    AllowedValues:
+      - One_Hour
+      - Three_Hours
+      - Six_Hours
+      - Twelve_Hours
+      - TwentyFour_Hours
 
 Resources:
   ValidateLambdaErrorTopic:
@@ -100,5 +110,5 @@ Resources:
         SourceDetails:
         - EventSource: "aws.config"
           MessageType: "ScheduledNotification"
-          MaximumExecutionFrequency: Three_Hours
+          MaximumExecutionFrequency: !Ref ValidationFrequency
         SourceIdentifier: !GetAtt ValidateStackSetDeploymentsLambdaFunction.Arn