fix type assertions in createBridge

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
rename 'default0' hardcoding to 'default', handle suffix digits
appropriately in name checker.

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
use name and interface less interchangably because it breaks like it
should now

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
Merge pull request #128 from erikh/fixups

Fixups