This fix fixes issue raised in 29492 where it was not
possible to specify a default `--default-shm-size` in daemon
configuration for each `docker run``.
The flag `--default-shm-size` which is reloadable, has been
added to the daemon configuation.
Related docs has been updated.
This fix fixes 29492.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>