Make the DB version configurable and set it to 5.6 by default.

cloudformation_templates/edx-reference-architecture.json

@@ -392,6 +392,16 @@
       ],
       "ConstraintDescription":"must select a valid database instance type."
     },
+    "DBEngineVersion":{
+      "Default":"5.6",
+      "Description":"Version of MySQL for the RDS",
+      "Type":"String",
+      "AllowedValues":[
+        "5.5",
+        "5.6"
+      ],
+      "ConstraintDescription":"must select a valid database version."
+    },
     "DBAllocatedStorage":{
       "Default":"5",
       "Description":"The size of the database (Gb)",
@@ -4369,7 +4379,7 @@
           "Ref":"DBClass"
         },
         "Engine":"MySQL",
-        "EngineVersion":"5.5",
+        "EngineVersion":{ "Ref": "DBEngineVersion" },
         "MasterUsername":{
           "Ref":"DBUsername"
         },