Handle openflowplugin returning null

It turns out that openflow plugin reports success with null transaction
ID if the switch does not support a feature. This is quite bad, but we
can recover rather easily. Rework the registration contract to take care
of the changes.

Change-Id: I62fe7badeb5d792c8533762f5e68b7d37811f8ed
Signed-off-by: Robert Varga <rovarga@cisco.com>