Install and configure a compute node in zun

[Francois Michaut]

This bug tracker is for errors with the documentation, use the following as a template and remove or add fields as you see fit. Convert [ ] into [x] to check boxes:

- [ ] This doc is inaccurate in this way: ______
- [x] This is a doc addition request.
- [ ] I have a fix to the document that I can paste below including example: input and output.

On the compute node installation manual of Zun, the systemd unity is defined as follow:

```
[Unit]
Description = OpenStack Container Service Compute Agent

[Service]
ExecStart = /usr/local/bin/zun-compute
User = zun

[Install]
WantedBy = multi-user.target
```

The issue with this unit, is that when I power on my machine, the unit boots before docker, and fails to connect with the following error :

```
 14:45:36.750 1078 ERROR zun File "/usr/local/lib/python3.8/dist-packages/docker/api/daemon.py", line 95, in info
 14:45:36.750 1078 ERROR zun return self._result(self._get(self._url("/info")), True)
 14:45:36.750 1078 ERROR zun File "/usr/local/lib/python3.8/dist-packages/docker/utils/decorators.py", line 46, in i>
 14:45:36.750 1078 ERROR zun return f(self, *args, **kwargs)
 14:45:36.750 1078 ERROR zun File "/usr/local/lib/python3.8/dist-packages/docker/api/client.py", line 237, in _get
 14:45:36.750 1078 ERROR zun return self.get(url, **self._set_request_timeout(kwargs))
 14:45:36.750 1078 ERROR zun File "/usr/lib/python3/dist-packages/requests/sessions.py", line 555, in get
 14:45:36.750 1078 ERROR zun return self.request('GET', url, **kwargs)
 14:45:36.750 1078 ERROR zun File "/usr/lib/python3/dist-packages/requests/sessions.py", line 542, in request
 14:45:36.750 1078 ERROR zun resp = self.send(prep, **send_kwargs)
 14:45:36.750 1078 ERROR zun File "/usr/lib/python3/dist-packages/requests/sessions.py", line 655, in send
 14:45:36.750 1078 ERROR zun r = adapter.send(request, **kwargs)
 14:45:36.750 1078 ERROR zun File "/usr/lib/python3/dist-packages/requests/adapters.py", line 498, in send
 14:45:36.750 1078 ERROR zun raise ConnectionError(err, request=request)
 14:45:36.750 1078 ERROR zun requests.exceptions.ConnectionError: ('Connection aborted.', PermissionError(13, 'EACCES'>
 14:45:36.750 1078 ERROR zun
ce: Main process exited, code=exited, status=1/FAILURE
```

Adding `After=docker.service` in the [Unit] section of the zun-compute systemd unit solves the issue. The documentation should contain the `After=docker.service` in the unit to avoid the issue.

-----------------------------------
Release: 7.0.0.0rc2.dev2 on 2020-04-22 00:51:11
SHA: 12685be9b4947d856a405fd059f0fe15aa957512
Source: https://opendev.org/openstack/zun/src/doc/source/install/compute-install.rst
URL: https://docs.openstack.org/zun/wallaby/install/compute-install.html