Respect custom gitlab_shell path in gitlab:gitlab_shell:check task

Prior to this change, if a user customized the config value of
`gitlab_shell.path`, the check would ignore this value in favor of
`"~<gitlab_shell.ssh_user>/gitlab-shell/"` resulting in a failed check
when the configuration was otherwise valid.

Why?

I develop on OSX, and after a recent reformat I found it way more difficult to
get a development environment for Gitlab running than it should have been. I
finally found a workable solution that involved no virtual machines and no
additional dedicated user, but this minor change was necessary to facilitate
it. I'll be writing a blog post a bit later describing what I finally got set
up.

Unless this seriously breaks something else, a merge would be appreciated.

Thanks!