Description
Docker daemon won't start when daemon.json contains ipv6: true.
The error is:

Error starting daemon: Error initializing network controller: Error creating default "bridge" network: failed to parse pool request for address space "LocalDefault" pool " subpool ": could not find an available, non-overlapping IPv6 address pool among the defaults to assign to the network

It was working just fine before the today's apt-get update.

Steps to reproduce the issue:

1. Add daemon.json to /etc/docker/ with { "ipv6": true }
2. Restart docker deamon.

Describe the results you received:
Daemon starting fails with error

Error starting daemon: Error initializing network controller: Error creating default "bridge" network: failed to parse pool request for address space "LocalDefault" pool " subpool ": could not find an available, non-overlapping IPv6 address pool among the defaults to assign to the network

Describe the results you expected:
Daemon should start just fine.

Additional information you deem important (e.g. issue happens only occasionally):
It worked before the latest update with a bunch of containers running.

Output of docker version:

Client:
 Version:      1.12.4
 API version:  1.24
 Go version:   go1.6.4
 Git commit:   1564f02
 Built:        Tue Dec 13 00:08:34 2016
 OS/Arch:      linux/amd64

Server:
 Version:      1.12.4
 API version:  1.24
 Go version:   go1.6.4
 Git commit:   1564f02
 Built:        Tue Dec 13 00:08:34 2016
 OS/Arch:      linux/amd64

Output of docker info:

Containers: 15
 Running: 3
 Paused: 0
 Stopped: 12
Images: 22
Server Version: 1.12.4
Storage Driver: aufs
 Root Dir: /var/lib/docker/aufs
 Backing Filesystem: extfs
 Dirs: 150
 Dirperm1 Supported: true
Logging Driver: json-file
Cgroup Driver: cgroupfs
Plugins:
 Volume: local
 Network: overlay bridge null host
Swarm: inactive
Runtimes: runc
Default Runtime: runc
Security Options: apparmor seccomp
Kernel Version: 4.4.0-53-generic
Operating System: Ubuntu 16.04.1 LTS
OSType: linux
Architecture: x86_64
CPUs: 1
Total Memory: 983.8 MiB
Name: kauchuk
ID: ROJ7:X3EK:6OVR:LI3P:TMVE:W44H:DLHN:SIGT:M6TN:O2OJ:3NQC:5FUG
Docker Root Dir: /var/lib/docker
Debug Mode (client): false
Debug Mode (server): false
Registry: https://index.docker.io/v1/
WARNING: No swap limit support
Insecure Registries:
 127.0.0.0/8

Additional environment details (AWS, VirtualBox, physical, etc.):
Docker is running on Ubuntu 16.04. Nothing else is installed.