Use features and desires to make sure that users get a node that
supports the OS they are asking for. Puts features 'OS-<osid>' on each pnode, listing the OSes that the pnode supports, and a desire on each vnode with a weight of 1 for the OS the user wants to run. This way, assign will not accidentally pick a node (such as a wireless PC) that the user's image will not run on. Note: This makes ptop files much, much larger, and makes assign take somewhat longer to run.