Also, I had to change the linuxbridge configuration around.  I'd been
using statically-configured linux bridges, and I'd assumed I could tell
Neutron to use them.  Unfortunately, this is sort of true, but not
enough; from the comments:

 NB: We can only control the name of the external br-ex bridge,
 because only the Neutron linuxbridge driver accepts both a map of
 physical networks to physical interfaces; and physical network to
 bridge names.  Nova assumes that the bridge it must plug a device
 into is named according to the physical network uuid.  Thus, for
 the linuxbridge case, we only setup bridge_mappings for
 br-ex... modulo a flag.  Hopefully in the future they will see the
 sense in allowing static bridge configurations.

So for now, only br-ex is static; the others are dynamic.  What a pain
for debugging!  Stupid.

More importantly, this commit also takes integrates correctly with the
new Emulab clientside improvements that let...