fix loginshell so that cloning/pushing via git/ssh works again

Finally, this was the cause why pushing via git protocol did not work.

I remember we set the login shell to `/bin/false` intentionally while we translated `sudo adduser --disabled-login --gecos 'GitLab' git` from the [installation instructions](https://github.com/gitlabhq/gitlabhq/blob/master/doc/install/installation.md) into a user resource.

But `--disabled-login` does not translate into `shell '/bin/false'` obviously :-)