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.