Allowing vnode experiments running on shared nodes to share IPv4 subnets
is a natural thing in Emulab; not so in Docker.  This is the Emulab side
of that fix.

The Docker side relies on modifications to Docker (see
https://gitlab.flux.utah.edu/emulab/moby/commits/emulab) to add a real
Layer 2 mode to the bridge and macvlan drivers, so that Docker doesn't
require IPv4 configuration of the bridge; and so that it allows Layer
2-only bridges to have overlapping IP subnets.  Normally it does not
allow the latter because it expects that any network might be
routable/reachable by any other network.