CentOS: Remove docker network bridge default

This disables the docker network bridge that is created by default
when no bridge options are provided by docker.service or daemon.json.
Since docker bridge is not used, it can be safely removed.

The docker.service file is provided by RPM docker-ce, i.e.,
rpm -q --whatprovides /usr/lib/systemd/system/docker.service
docker-ce-18.09.6-3.el7.x86_64

This file contains the default ExecStart:
[Service]
ExecStart=/usr/bin/dockerd -H fd:// \
 --containerd=/run/containerd/containerd.sock

The ExecStart gets overridden by a Drop-In. The previous default
setting gets wiped out using "ExecStart=", then the value is redefined
with same options and "--bridge=none" appended.
  Drop-In: /etc/systemd/system/docker.service.d
           └─docker-stx-override.conf

If the network with address 172.17.0.0/16 (or similar network) and it
gateway address is 172.17.0.1, this IP address causes conflict with
docker0 bridge. This results in packet loss between GW and application
pods.

Closes-Bug: 1996916

Test Plan:
PASS: AIO-SX Fresh install ISO. Verify docker bridge not configured.
      i.e., 'sudo docker network ls'
PASS: Designer in-service patch apply and remove (with this change).
      Verify docker bridge not configured.
      i.e., 'sudo docker network ls'

Signed-off-by: Jim Gauld <james.gauld@windriver.com>
Change-Id: Ibd0164002744f1bd56e14fdb53c5b9a935b1fcc4

docker-config/files/docker-stx-override.conf

@@ -1,4 +1,6 @@
 [Service]
+ExecStart=
+ExecStart=/usr/bin/dockerd -H fd:// --containerd=/run/containerd/containerd.sock --bridge=none
 ExecStartPost=/bin/bash -c 'echo $MAINPID > /var/run/dockerd.pid;'
 ExecStopPost=/bin/rm -f /var/run/dockerd.pid