ovs-dpctl: Add a 'filter' option to match wildcarded 'dump-flows'.

ovs-dpctl: Add a 'filter' option to match wildcarded 'dump-flows'.

With mega-flows, many flows in the kernel datapath are wildcarded.
For someone that is debugging a system and wants to find a particular
flow and its actions, it is a little hard to zero-in on the flow
because some fields are wildcarded.

With the filter='$filter' option, we can now filter on the o/p
of 'ovs-dpctl dump-flows'.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>