Posibility to create IPv6 only network

[DennisGlindhart]

Description

Trying to create a IPv6-only network, but cannot prevent Docker from assigning IPv4 subnet/address as it have defaults.
It should be possible somehow to disable the IPv4-assignment (or document how it is already possible).

Steps to reproduce the issue:
docker network create -d macvlan --subnet=2001:db8::/64 --ipv6 --gateway=2001:db8::1 -o parent=eth0 example

Describe the results you received:
docker network inspect example

   {
        "Name": "pub",
        "Scope": "local",
        "Driver": "macvlan",
        "EnableIPv6": true,
        "IPAM": {
            "Driver": "default",
            "Options": {},
            "Config": [
                {
                    "Subnet": "172.19.0.0/16",
                    "Gateway": "172.19.0.1"
                },
                {
                    "Subnet": "2001:db8::/64",
                    "Gateway": "2001:db8::1"
                }
            ]
        },
        "Internal": false,
        "Attachable": false,
        "Ingress": false,
        "ConfigFrom": {
            "Network": ""
        },
        "ConfigOnly": false,
        "Containers": {},
        "Options": {
            "parent": "eth0"
        },
        "Labels": {}
    }

Describe the results you expected:

IPv4-subnet definition should be gone.

docker network inspect example

   {
        "Name": "pub",
        "Scope": "local",
        "Driver": "macvlan",
        "EnableIPv6": true,
        "IPAM": {
            "Driver": "default",
            "Options": {},
            "Config": [
                {
                    "Subnet": "2001:db8::/64",
                    "Gateway": "2001:db8::1"
                }
            ]
        },
        "Internal": false,
        "Attachable": false,
        "Ingress": false,
        "ConfigFrom": {
            "Network": ""
        },
        "ConfigOnly": false,
        "Containers": {},
        "Options": {
            "parent": "eth0"
        },
        "Labels": {}
    }

Additional information you deem important (e.g. issue happens only occasionally):

Host does not have IPv4 address (Except on lo-interface)

Output of docker version:

Client:
 Version:           18.06.1-ce
 API version:       1.38
 Go version:        go1.10.3
 Git commit:        e68fc7a
 Built:             Tue Aug 21 17:23:03 2018
 OS/Arch:           linux/amd64
 Experimental:      false

Server:
 Engine:
  Version:          18.06.1-ce
  API version:      1.38 (minimum version 1.12)
  Go version:       go1.10.3
  Git commit:       e68fc7a
  Built:            Tue Aug 21 17:25:29 2018
  OS/Arch:          linux/amd64
  Experimental:     false

Output of docker info:

Containers: 0
 Running: 0
 Paused: 0
 Stopped: 0
Images: 1
Server Version: 18.06.1-ce
Storage Driver: overlay2
 Backing Filesystem: xfs
 Supports d_type: true
 Native Overlay Diff: true
Logging Driver: json-file
Cgroup Driver: cgroupfs
Plugins:
 Volume: local
 Network: bridge host macvlan null overlay
 Log: awslogs fluentd gcplogs gelf journald json-file logentries splunk syslog
Swarm: inactive
Runtimes: runc
Default Runtime: runc
Init Binary: docker-init
containerd version: 468a545b9edcd5932818eb9de8e72413e616e86e
runc version: 69663f0bd4b60df09991c08812a60108003fa340
init version: fec3683
Security Options:
 seccomp
  Profile: default
Kernel Version: 3.10.0-862.11.6.el7.x86_64
Operating System: CentOS Linux 7 (Core)
OSType: linux
Architecture: x86_64
CPUs: 2
Total Memory: 11.57GiB
Name: xxx
ID: GAS6:X44Z:MIPV:QW6A:3AUX:IFGG:Y6AB:CGT6:Q3ED:3EED:FWT4:YEM5
Docker Root Dir: /var/lib/docker
Debug Mode (client): false
Debug Mode (server): false
Registry: https://index.docker.io/v1/
Labels:
Experimental: false
Insecure Registries:
 127.0.0.0/8
Live Restore Enabled: false

[agowa]

Relates to: moby/libnetwork#826