CC-3528: SaaS compatible upgrade script template

-Done

install_minimal/upgrades/airtime-2.1.0/DbUpgrade.php

@@ -0,0 +1,17 @@
+<?php
+
+/* All functions other than start() should be marked as
+ * private.
+ */
+class AirtimeDatabaseUpgrade{
+
+    public static function start(){
+        echo "* Updating Database".PHP_EOL;
+        self::task0();
+    }
+
+    private static function task0(){
+        
+    }
+
+}