Add `--stop-signal` for `service create` and `service update`

This fix tries to address the issue raised in 25696 where
it was not possible to specify `--stop-signal` for `docker service create`
and `docker service update`, in order to use special signal to stop
the container.

This fix adds `--stop-signal` and update the `StopSignal` in `Config`
through `service create` and `service update`.

Related docs has been updated.

Integration test has been added.

This fix fixes 25696.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

docs/reference/commandline/service_create.md

@@ -58,6 +58,7 @@ Options:
       --restart-window duration          Window used to evaluate the restart policy (ns|us|ms|s|m|h)
       --secret secret                    Specify secrets to expose to the service
       --stop-grace-period duration       Time to wait before force killing a container (ns|us|ms|s|m|h)
+      --stop-signal string               Signal to stop the container
   -t, --tty                              Allocate a pseudo-TTY
       --update-delay duration            Delay between updates (ns|us|ms|s|m|h) (default 0s)
       --update-failure-action string     Action on update failure ("pause"|"continue") (default "pause")

docs/reference/commandline/service_update.md

@@ -70,6 +70,7 @@ Options:
       --secret-add secret                Add or update a secret on a service
       --secret-rm list                   Remove a secret (default [])
       --stop-grace-period duration       Time to wait before force killing a container (ns|us|ms|s|m|h)
+      --stop-signal string               Signal to stop the container
   -t, --tty                              Allocate a pseudo-TTY
       --update-delay duration            Delay between updates (ns|us|ms|s|m|h) (default 0s)
       --update-failure-action string     Action on update failure ("pause"|"continue") (default "pause")