nvp plugin rxtx_factor readonly update port

nvp plugin rxtx_factor readonly update port

The following change done in nova 7de916 started passing in the
rxtx_factor on update port rather than just on create_port which
is what we only originally supported. Therefore currently when
booting a vm and specifying --nic port-id it will fail to boot with:
Cannot update read-only attribute rxtx_factor. This patch is a work
around to allow the rxtx_factor value to be passed in on update port even
though we ignore the value. Later we'll implement updating rxtx_factor on
ports but this is a good work around for now.

Fixes bug: 1202406

Change-Id: Iedd488b7bdc9b1a1317000d249f03b0eafbea419