[provisioners/chef] Support Chef Solo machine nodes

The ability to override the `node_name` in the base configuration of both Chef provisioners has been around for awhile. This commit adds ability to override a path to the a directory full of machine nodes and it subsequently uses the same rsync mechanism so that Chef Solo on the guest machine can use this directory for provisioning.

Right now this seems to be limited by [how the nodes are loaded in Chef](https://github.com/opscode/chef/blob/master/lib/chef/client.rb#L346) but I have been tweaking some custom solo.rb  files and passing them into chef-solo and chef-client. The local mode, assuming Chef zero, seems to work fine.