Gracefully handle network bridge without IP association at startup

Addresses #8444

Docker-DCO-1.1-Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com> (github: estesp)
It would be nice to have a cleaner way to check for bridge existence, and I looked into that first before ending up with this solution.  At this point, to do something more like a bridge existence check rather than test for error we would need a new API in libcontainer netlink layer similar to what you can do with ```brctl show``` to query a bridge.

The "file exists" response is a well-known return from the RTNETLINK layer, not something from Go and/or Docker, so is presumed quite stable as far as depending on it to not change in Linux as a syscall response.