This fix tries to address the issue raised in 29972 where
it was not possible to specify `--read-only` for `docker service create`
and `docker service update`, in order to have the container's root file
system to be read only.
This fix adds `--read-only` and update the `ReadonlyRootfs` in `HostConfig`
through `service create` and `service update`.
Related docs has been updated.
Integration test has been added.
This fix fixes 29972.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>