Add flag `--host` to `service create` and `--host-add/--host-rm` to `service update`

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>

docs/reference/commandline/service_create.md

@@ -35,6 +35,7 @@ Options:
       --health-retries int               Consecutive failures needed to report unhealthy
       --health-timeout duration          Maximum time to allow one check to run (default none)
       --help                             Print usage
+      --host list                        Set one or more custom host-to-IP mappings (host:ip) (default [])
       --hostname string                  Container hostname
   -l, --label list                       Service labels (default [])
       --limit-cpu decimal                Limit CPUs (default 0.000)

docs/reference/commandline/service_update.md

@@ -43,6 +43,8 @@ Options:
       --health-retries int               Consecutive failures needed to report unhealthy
       --health-timeout duration          Maximum time to allow one check to run (default none)
       --help                             Print usage
+      --host-add list                    Add or update a custom host-to-IP mapping (host:ip) (default [])
+      --host-rm list                     Remove a custom host-to-IP mapping (host:ip) (default [])
       --image string                     Service image tag
       --label-add list                   Add or update a service label (default [])
       --label-rm list                    Remove a label by its key (default [])