Default to 3 mongo instances so that clusters do not need an arbiter.

cloudformation_templates/edx-reference-architecture.json

@@ -235,7 +235,7 @@
     "MongoDesiredCapacity":{
       "Description":"The Auto-scaling group desired capacity for the mongodb hosts",
       "Type":"Number",
-      "Default":"2"
+      "Default":"3"
     },
     "CacheNodeType":{
       "Default":"cache.m1.small",
@@ -5060,8 +5060,8 @@
         "LaunchConfigurationName":{
           "Ref":"MongoServer"
         },
-        "MinSize":"2",
-        "MaxSize":"2",
+        "MinSize":"3",
+        "MaxSize":"3",
         "DesiredCapacity":{
           "Ref":"MongoDesiredCapacity"
         }