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
Nicira plugin: always fetch lports from correct lswitch

Bug 1188652

Remove get_logical_port_status which required a lswitch uuid, and
replace it with a call to query_lswitch_ports, which does not
require it.

Change-Id: I6c7d0403f37b2c9fb4332a8d34dcaa9050ced3fc
Merge "Nicira plugin: always fetch lports from correct lswitch"
Merge "nicira plugin: load subnet once when adding router interface"