Add support for stop-signal in stack command

[christopherobin]

- What I did

Fixes #370 by passing StopSignal to the container spec when doing a deploy.

- How I did it

Passed StopSignal and removed stop_signal from unsupported keys.

- How to verify it

Create a small docker-compose file with a service defining a stop signal, deploy using docker stack deploy -c file.yml stack_name then inspect one of the created containers.

- Description for the changelog

Added support for stop_signal when deploying a compose file through docker stack.

- A picture of a cute animal (not mandatory but encouraged)

[codecov-io]

Codecov Report

Merging #388 into master will decrease coverage by 0.34%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #388      +/-   ##
==========================================
- Coverage   46.14%   45.79%   -0.35%     
==========================================
  Files         193      193              
  Lines       16073    16070       -3     
==========================================
- Hits         7417     7360      -57     
- Misses       8269     8324      +55     
+ Partials      387      386       -1

[dnephin]

LGTM

[vdemeester]

LGTM 🐯

[thaJeztah]

ping @londoncalling to verify if this needs docs updates

[londoncalling]

@thaJeztah , it looks like we have this in the Compose file reference, but currently say it's ignored by docker stack deploy: https://docs.docker.com/compose/compose-file/#stop_signal

Is the first part of this docs update to remove the caveat here re: docker stack deploy (as it's now supported)? And update the versioning information?

Regarding Compose CLI support, I assume this warrants a new Compose CLI command topic?