bridge: Keep default Ethernet address stable between runs.

bridge: Keep default Ethernet address stable between runs.

In some circumstances the bridge can't find a stable physical Ethernet
address to use, so until now it has just picked a random Ethernet address.
In these circumstances, therefore, the bridge Ethernet address would change
from one ovs-vswitchd run to another.  But OVS does have a stable
identifier for a bridge: its UUID.  This commit changes to use that as the
default bridge Ethernet address.

The datapath ID is sometimes derived from the bridge Ethernet address, so
this change also makes the bridge Ethernet address more stable.

CC: Natasha Gude <natasha@nicira.com>
Bug #5594.