Description:
docker0 network interface doesn´t change de MTU settings according to /etc/docker/daemon.json file.

Steps to reproduce the issue:

1. Create a /etc/docker/daemon.json file with below content:

{ "bip": "172.17.0.200/24", "mtu": 1400 }

2. Restart the docker service;
3. Check docker0 interface MTU settings.

Describe the results you received:
docker0 interface remains with the default MTU configuration (1500).

[xxx@yyy ~]$ sudo ifconfig docker0
docker0: flags=4099<UP,BROADCAST,MULTICAST> mtu 1500
inet 172.17.0.200 netmask 255.255.255.0 broadcast 172.17.0.255
ether 02:42:1c:cb:b1:be txqueuelen 0 (Ethernet)
RX packets 0 bytes 0 (0.0 B)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 0 bytes 0 (0.0 B)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

Describe the results you expected:
I expected to update the mtu value of docker0 interface after restarting the service.

Additional information you deem important (e.g. issue happens only occasionally):
I´ve change the bip just to make sure that dockerd is reading the /etc/docker/daemon.json file.
I´m running docker on a server over OpenStack. The configuration of MTU is mandatory to work without huge performance impacts.

Output of docker version:
[xxx@yyy ~]$ sudo 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:

[xxx@yyy ~]$ sudo docker info
Containers: 0
Running: 0
Paused: 0
Stopped: 0
Images: 0
Server Version: 18.06.1-ce
Storage Driver: devicemapper
Pool Name: docker-8:1-50437250-pool
Pool Blocksize: 65.54kB
Base Device Size: 10.74GB
Backing Filesystem: xfs
Udev Sync Supported: true
Data file: /dev/loop0
Metadata file: /dev/loop1
Data loop file: /var/lib/docker/devicemapper/devicemapper/data
Metadata loop file: /var/lib/docker/devicemapper/devicemapper/metadata
Data Space Used: 11.73MB
Data Space Total: 107.4GB
Data Space Available: 51.7GB
Metadata Space Used: 581.6kB
Metadata Space Total: 2.147GB
Metadata Space Available: 2.147GB
Thin Pool Minimum Free Space: 10.74GB
Deferred Removal Enabled: true
Deferred Deletion Enabled: true
Deferred Deleted Device Count: 0
Library Version: 1.02.146-RHEL7 (2018-01-22)
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: 3.10.0-862.14.4.el7.x86_64
Operating System: CentOS Linux 7 (Core)
OSType: linux
Architecture: x86_64
CPUs: 8
Total Memory: 15.51GiB
Name: SUPRESSED BY MYSELF
ID: UTZ5:7H4Q:I7YK:F43L:B5HQ:Q24M:RJIR:7ZKR:DOAN:Y65M:M4KK:PTDY
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

WARNING: devicemapper: usage of loopback devices is strongly discouraged for production use.
Use --storage-opt dm.thinpooldev to specify a custom block storage device.