postfix: allow list for virtual alias destinations

postfix/templates/virtual.j2

@@ -9,7 +9,7 @@ abuse@{{ domain }} {{ adminaddr }}
 {{ alias.src }} {{ alias.dest }}
 {% else %}
 {% for src in alias.src %}
-{{ src }}@{{ alias.domain }} {{ alias.dest }}
+{{ src }}@{{ alias.domain }} {{ alias.dest if alias.dest is string else alias.dest|join(', ') }}
 {% endfor %}
 {% endif %}
 {% endfor %}