Allow admins full ssh access

This depends on a new GitLab internal api call "admin" to
check if the user in an admin.

If the user is an admin, then gitlab-shell should become transparent.

This allows capistrano to continue to work while GitLab is running.

If GitLab isn't running, then it will prevent admin access (alas).

I also replaced `system()` with `Kernel::exec()` because there is
no reason to hang around.