Veth pairs standing up, adding to bridge

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
simplebridge: start of tests and some fixes along the way

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
simplebridge: only create network/<network>/bridgeInterface once

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
Merge pull request #112 from erikh/network-extensions

Network extensions