Control resource extension process defining required extensions

Control resource extension process defining required extensions

Blueprint l3-ext-gw-modes

The resource extension process is slightly altered so that an
API extension can specify which extensions should be processed before
its attributes can be processed. This is needed to ensure the
l3-ext-gw-mode API extension sets up correctly validators and other
attributes for the external_gateway_info attribute

Change-Id: I7c05746e67b1324a73082dde8b5c862d631c083c