Add missing interface to populate subnets method

Add missing interface to populate subnets method

Change Ib46f685d72eb61ecbaa2869e28fb173cd6d49552 introduced
and optimization to defer the lookup of interface subnet info
until all of the router interfaces were collected. However,
it didn't add the DVR SNAT interface to the list of interfaces
to populate subnet info so it broke DVR.

This patch corrects the behavior by adding the DVR SNAT interface
to the list of ports that need subnet info populated.

Change-Id: I32054ff00bf6992c5dedd21735b6d2afd15c5fb3