Add support for stop_signal in docker stack deploy

[christopherobin]

Description

stop_signal support was added to swarmkit in docker/swarmkit#1924 and docker in moby/moby#30754 but it is still not supported via docker stack deploy -c compose-file.yml stack_name.

It would be a welcome addition since some of our containers need SIGINT to properly exit.
If it's fine with you guys I could probably do a PR with this functionality.

Steps to reproduce the issue:

1. Add stop_signal: SIGINT to a docker-compose file
2. docker stack deploy -c <file> <name>
3. docker stack scale service=0

Describe the results you received:

The service gets a SIGTERM signal

Describe the results you expected:

The service gets a SIGINT signal

Additional information you deem important (e.g. issue happens only occasionally):

Output of docker version:

Client:
 Version:      17.06.0-ce
 API version:  1.30
 Go version:   go1.8.3
 Git commit:   3dfb8343
 Built:        Sat Jul  8 20:56:41 2017
 OS/Arch:      linux/amd64

Server:
 Version:      17.06.0-ce
 API version:  1.30 (minimum version 1.12)
 Go version:   go1.8.3
 Git commit:   02c1d87617
 Built:        Sat Jul  8 22:56:56 2017
 OS/Arch:      linux/amd64
 Experimental: false

Output of docker info:

N/A

Additional environment details (AWS, VirtualBox, physical, etc.):

N/A

[thaJeztah]

Support for stop-signal was added to services in docker 17.04 through moby/moby#30754, and moby/moby#31091, so this should just be adding the option to the docker-compose file specs and cli

[salzig]

https://docs.docker.com/compose/compose-file/#not-supported-for-docker-stack-deploy

The Documentation is still telling us another story.

[thaJeztah]

@salzig thanks! Looks like the documentation side changes never found their way in; see
#388 (comment)

Are you interested in opening a pull request to make that change? There's an "edit this page" link on the top-right of that page in the documentation; that should open an editor on GitHub and you can open a pull-request from that 🤗