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)
If there is worry about destroying the environment, a simple ```brctl delbr docker0``` added to the failure path will make sure that any future daemon starts will work even without the fix for this issue.  However, I wasn't sure about the environment this test is running in--whether I could/should depend on ```brctl``` being available. Another option is to save the IP and restore in the failure path.