fix(server): fix `salt-lint` violations [skip ci]

```console
[206] Jinja variables should have spaces before and after: {{ var_name }}
mysql/server.sls:86
        {{debconf_root_password}}: {'type': 'password', 'value': '{{ mysql_root_password }}'}

[206] Jinja variables should have spaces before and after: {{ var_name }}
mysql/server.sls:87
        {{debconf_root_password_again}}: {'type': 'password', 'value': '{{ mysql_root_password }}'}
```