fix(server.sls): fix `salt-lint` errors

```bash
Examining mysql/server.sls of type state
[206] Jinja variables should have spaces before and after: {{ var_name }}
mysql/server.sls:119
    - creates: {{ mysql_datadir}}/mysql/

[206] Jinja variables should have spaces before and after: {{ var_name }}
mysql/server.sls:141
    - creates: {{ mysql_datadir}}/mysql/
```