Core extensions management

Signed-off-by: Arnaud Porterie <arnaud.porterie@docker.com>
Merge pull request #111 from icecrime/extensions

More core adaptations
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)