Use the gitlab-shell path from the config in Gitlab:check

This fixes an issue in the self check, if you have Gitlab-shell in a subdirectory, the check would fail, because it would only check for`~/#{gitlab_shell_user_home}/gitlab-shell`. 

But when you have it in an subdirectory it had to check like:
`~/#{gitlab_shell_user_home}/#{sub_dir}/gitlab-shell`