Commit Ib56ee8bfd182c031e468c503acb0cd75daea8c40 refactored code
in L2 gateway base plugin. This patch removes unused NSX plugin
code and makes appropriate changes to NSX-V driver.
This patch also adds a couple of update_l2_gateway methods to
both NSX|V and NSX|V3 L2GW drivers. These methods are added
since the base plugin expects the driver to define them.
Change-Id: Ifda5401ec5134268b8a9c3276ed7abf4cf8874aa
Partial-Bug: #1591413
ecbed5d940