Account dcoker0 IPv6 address on daemon start

- In case --fixed-cidr-v6 is specified and docker0 bridge already
  has a global scope IPv6 address belonging to that v6 network
  (likely from a previous daemon instance), to maintain consistency
  with what done for the docker0 IPv4 address, daemon has to pass it
  down to libnetwork in the IPAMConfig as network gateway to make
  sure that the address is not given to some container.

Signed-off-by: Alessandro Boch <aboch@docker.com>