This adds both a daemon-wide flag and a container creation property:
- Set the `CgroupnsMode: "host|private"` HostConfig property at
container creation time to control what cgroup namespace the container
is created in
- Set the `--default-cgroupns-mode=host|private` daemon flag to control
what cgroup namespace containers are created in by default
- Set the default if the daemon flag is unset to "host", for backward
compatibility
- Default to CgroupnsMode: "host" for client versions < 1.40
Signed-off-by: Rob Gulewich <rgulewich@netflix.com>
