Change EventScheduleRate parameter to EventSchedulePeriod and fix rat…

…e expression

The rate expression must pluralise the unit, so `1 minute` and `0 minutes` are valid, but `0 minute` is not.

template.yaml

@@ -33,9 +33,9 @@ Parameters:
     Type: String
     Default: ''
 
-  EventScheduleRate:
+  EventSchedulePeriod:
     Description: How often the Event Schedule is triggered (in minutes)
-    Type: String
+    Type: Number
     Default: "1"
 
   AgentsPerInstance:
@@ -89,7 +89,7 @@ Parameters:
     Type: Number
     Description: The number of pages to retrive for DescribeScalingActivity. Negative numbers mean unlimited.
     Default: "-1"
-  
+
   MinPollInterval:
     Type: String
     Description: Minimum time interval between polls. If a larger interval is provided by Buildkite, that is used instead.
@@ -102,6 +102,8 @@ Conditions:
     !Not [ !Equals [ !Ref BuildkiteAgentTokenParameterStoreKMSKey, "" ] ]
   SetRolePermissionsBoundaryARN:
     !Not [ !Equals [ !Ref RolePermissionsBoundaryARN, "" ] ]
+  EventSchedulePeriodIsOne:
+    !Equals [ !Ref EventSchedulePeriod, "1" ]
 
 Mappings:
   LambdaBucket:
@@ -213,7 +215,7 @@ Resources:
         Timer:
           Type: Schedule
           Properties:
-            Schedule: !Sub "rate(${EventScheduleRate} minute)"
+            Schedule: !If [ EventSchdulePeriodIsOne, "rate(1 minute)", !Sub "rate(${EventSchedulePeriod} minutes)"]
 
   # This mirrors the group that would be created by the lambda, but enforces
   # a retention period and also ensures it's removed when the stack is removed