Recent change If2fe1e014b78703ff0a9cdff1e4e8d45f3a4a16d
broke L2GW driver for NSXv3. This patch makes sure that the driver
starts and creates bridge clusters using the new nsxlib.
This patch adds a callback to create default l2 gw after the core
plugin is initialized.
Change-Id: Iced699b13e265f515c23fcc952ba9bb1e64b5762