Workaround for Docker 1.11.2 startup issue

On a percentage of DC/OS agents (~5%) with Docker 1.11.2, the Daemon
will fail to start up with the following error:

> Error starting daemon: Error initializing network controller: Error
> creating default "bridge" network: failed to allocate gateway
> (172.17.0.1): Address already in use

This seems to be related to a Docker bug around the network controller
initialization, where the controller has allocated an ip pool and
persisted some state but not all of it. See:

* moby/moby#22834
* moby/moby#23078

This fix simply removes the docker0 interface if it exists before
starting the Docker daemon. This fix will need to be re-evaluated if we
want to enable the 1.12+ containerd live-restore like Docker options as
discussed in:

* https://docs.docker.com/engine/admin/live-restore/
* moby/moby#2658

cloud_images/centos7/install_prereqs.sh

@@ -55,6 +55,10 @@ docker_service_d=/etc/systemd/system/docker.service.d
 mkdir -p "$docker_service_d"
 cat << 'EOF' > "${docker_service_d}/execstart.conf"
 [Service]
+Restart=always
+StartLimitInterval=0
+RestartSec=15
+ExecStartPre=-/sbin/ip link del docker0
 ExecStart=
 ExecStart=/usr/bin/docker daemon -H fd:// --graph=/var/lib/docker --storage-driver=overlay
 EOF

gen/azure/cloud-config.yaml

@@ -19,6 +19,7 @@ root:
       Restart=always
       StartLimitInterval=0
       RestartSec=15
+      ExecStartPre=-/sbin/ip link del docker0
       ExecStart=
       ExecStart=/usr/bin/docker daemon -H fd:// --storage-driver=overlay
   - path: /etc/systemd/system/docker.socket