CLOUDSTACK-6485 prevent ip asignment of private gw iface

Prevent ipaddress asignment of gateway to gateway-interface on vpc router by setting vpcid to null in network

Was fixed in 4.4 by 1f209ff226
Reimplemented for 4.7

server/src/com/cloud/network/vpc/VpcManagerImpl.java

@@ -1668,7 +1668,7 @@ public class VpcManagerImpl extends ManagerBase implements VpcManager, VpcProvis
                         // A more permanent solution would be to define a type of 'gatewaynetwork'
                         // so that handling code is not mixed between the two
                         final NetworkVO gatewaynet = _ntwkDao.findById(privateNtwk.getId());
-                        gatewaynet.setVpcId(vpcId);
+                        gatewaynet.setVpcId(null);
                         _ntwkDao.persist(gatewaynet);
                     }