This fixes the daemon's failure to start when setting --ipv6=true for
the first time without deleting `docker0` bridge from a prior use with
only IPv4 addressing.
The addition of the IPv6 bridge address is factored out into a separate
initialization routine which is called even if the bridge exists but no
IPv6 addresses are found.
Docker-DCO-1.1-Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com> (github: estesp)
Upstream-commit: 0c8d17b5c1a142bc09abe1105d985e76db6f225d
Component: engine
