SNM used to define virtual_alias_maps in extraConfig which collides with
the same parameter defined by the standard services.postfix.virtual
option. This led to *lots* of warnings during postfix startup like
```
May 02 18:29:58 nun postfix/master[24758]: warning: /etc/postfix/main.cf, line 47: overriding earlier entry: virtual_alias_maps=hash:/etc/postfix/virtual
```
Refraining from overriding virtual_alias_maps has the additional
advantage that virtual aliases defined by other modules dont' stop
working with SNM.
