support multiple sasl_passwd entries

with tests

Signed-off-by: Felix Wong <gnowxilef@gmail.com>

templates/sasl_passwd.erb

@@ -1,4 +1,8 @@
 # Auto-generated by Chef.
 # Local modifications will be overwritten.
-#
-<%= node['postfix']['main']['relayhost'] %> <%= @settings['smtp_sasl_user_name'] %>:<%= @settings['smtp_sasl_passwd'] %>
+
+<% if !@settings.nil? && !@settings.empty? -%>
+<%   @settings.sort.map do |relayhost,value| -%>
+<%= relayhost %> <%= value['username'] %>:<%= value['password'] %>
+<%   end -%>
+<% end -%>