update-port error if port does not exist in nvp

update-port error if port does not exist in nvp

Previously the NVP plugin would query NVP for the quantum port id that was
stored in the port as a tag in NVP. Later, a look up table was added in order
to directly look up the ID without having to go to NVP. The problem that can
arise is that if a port was created in folsom before this lookup table was
added and then deleted from NVP directly an error will be raised when calling
update_port(). This patch fixes this potential problem.

Fixes bug 1172006

Change-Id: I08c74e305449f7fe32897abf43ceb430c959b02d