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 the user customize interface
config and /etc/hosts file generation.  To handle hosts, we create a
static manifest file that tells Emulab to call an rc.hostnames
pre-hook.  This hook basically grabs our latest special openstack hosts
entries, and ensures they make it into /etc/hosts.head, so that
genhostsfile prepends our special names.  To handle interfaces, we
further customize /etc/network/interfaces so that Emulab's rc.ifconfig
only tries to configure interfaces we haven't handled.

Thus, if the clientside of the disk image the scripts are operating on
includes the new clientside hookable support, we no longer move
rc.ifconfig and rc.hostnames out of our way --- we let them run, secure
in the knowledge that our customizations won't get trampled.

(All this improvement was necessary so that blockstores and event system
stuff would work.)