nicira plugin: load subnet once when adding router interface

Bug 1188562

This patch ensure subnet is loaded only once from the database.
Also, the port is now loaded from the database only if the
corresponding NVP logical port is found.

Change-Id: I1626c7b61ddc8e785d8ce2e39619da78b20ae61e
Merge "nicira plugin: load subnet once when adding router interface"