Use openvswitch ports for seed vm's.

Linuxbridge control plane networks have some fundamental issue with
host->seed->vm traffic. This makes the default be an ovs bridge which
works better.

Partial fix for bug 1194700.
Add a setup-network.

bm-poseur is overly complex and hard to use, and we need to switch to
using ovs bridges rather than linuxbridge bridges.
Generalise bootstrap utilities.

Rather than having two copies of VM creation logic, have just one copy
in the incubator tree, and parameterise it to create both seed and bm
nodes. Further, add a small helper to create a number of bm-nodes.