networkd: turn UnicastFlood on by default

networkd: turn UnicastFlood on by default

Looking at the kernel commit, "on" seems to be the default value:
commit 867a59436fc35593ae0e0efcd56cc6d2f8506586
Author: Vlad Yasevich <vyasevic@redhat.com>
Date:   Wed Jun 5 10:08:01 2013 -0400

    bridge: Add a flag to control unicast packet flood.

    Add a flag to control flood of unicast traffic.  By default, flood is
    on and the bridge will flood unicast traffic if it doesn't know
    the destination.  When the flag is turned off, unicast traffic
    without an FDB will not be forwarded to the specified port.

... and it seems to be the reasonable thing to do by default.