Deprecate docker-ssh

We should deprecate docker-ssh executor with GitLab Runner 10.0.

It seems that the functionality implemented by that one are not widely used and it also has deficiencies as with other executors, like: ssh, parallels, virtualbox. It does require to install specific version of helper on the target system.

cc @bikebilly @tmaczukin

@bikebilly I'd vote to deprecate both - docker-ssh and docker-ssh+machine. docker-ssh+machine differs from docker+machine in the same way as docker-ssh from docker - they require an image that have SSH installed and configured to be started automatically. I think it could be popular 2 years ago, but today I think that people try to avoid using SSH inside of the container since it's discouraged by Docker authors and Docker have better ways to access container (like docker exec).

I agree here with @tmaczukin.

and release post about these deprecations

@bikebilly Do you plan a separated blog post about deprecating some features with 10.0, or it will be enough to add such information in 10.0 release post? For #2710 (closed) we've planned to add deprecation note in the release post, so I assume this one should be handled the same.

mentioned in issue #2752 (closed)

mentioned in merge request !681 (merged)

added Documentation and removed ~60197 labels