Add Nicira extension for modifying queue without transmitting

The OpenFlow OFPAT_ENQUEUE action sets a queue id and outputs the packet
in one shot.  There are times in which the queue should be set, but the
output port is not yet known.  This commit adds the NXAST_SET_QUEUE and
NXAST_POP_QUEUE Nicira extension actions to modify the queue
configuration without requiring a port argument.

CC: Jeremy Stribling <strib@nicira.com>
CC: Keith Amidon <keith@nicira.com>