This fix tries to address 27902 by adding a flag `--host`
to `docker service create` and `--host-add/--host-rm` to
`docker service update`, so that it is possible to
specify extra `host:ip` settings in `/etc/hosts`.
This fix adds `Hosts` in swarmkit's `ContainerSpec` so that it
is possible to specify extra hosts during service creation.
Related docs has been updated.
An integration test has been added.
This fix fixes 27902.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>