[COOK 3473] Allow augmentation of users on per machine basis

http://tickets.opscode.com/browse/COOK-3473

Frequently, root access or application user access ends up being the same across all machines in a chef org.

Sometimes though, it is necessary to give certain additional users access either to a specific machine or to a small subset of machines based on a role assigned to those machines. This feature aims to fulfill that purpose in an unobtrusive, controlled manner.