Add GSNContext.setDefaultRelayHub (#69)

* add GSNContext.setDefaultRelayHub

* change setDefaultRelayHub to revert if already run

contracts/GSN/GSNContext.sol

@@ -19,6 +19,12 @@ contract GSNContext is Initializable, Context {
     event RelayHubChanged(address indexed oldRelayHub, address indexed newRelayHub);
 
     function initialize() public initializer {
+        if (_getRelayHub() == address(0)) {
+            setDefaultRelayHub();
+        }
+    }
+
+    function setDefaultRelayHub() public {
         _upgradeRelayHub(0xD216153c06E857cD7f72665E0aF1d7D82172F494);
     }