introduced --subnet, --ip-range and --gateway options in docker network
command. Also, user can allocate driver specific ip-address if any using
the --aux-address option.
Supports multiple subnets per network and also sharing ip range
across networks if the network-driver and ipam-driver supports it.
Example, Bridge driver doesnt support sharing same ip range across
networks.
Signed-off-by: Madhu Venugopal <madhu@docker.com>