[COOK-2551] Support creating the sender_canonical map file

http://tickets.opscode.com/browse/COOK-2551

The cookbook supports telling Postfix where to find a sender_canonical map file, but not actually creating one. This patch adds a new attribute, sender_canonical_map, which if populated gets used to create a file in the location specified by sender_canonical_maps.

This is a duplicate of a previous request, rewritten to merge cleanly with the current master.