Use the full path to `chef-solo` and `chef-client`

This fixes a bug where a cookbook (like rbenv, rvm, chruby) sets the default system Ruby. Chruby and RBenv specifically use $PATH munging, making those binaries unavailable on subsequent Test Kitchen runs.

Since other people can "touch" `/usr/local`, assuming the symlinks in `/usr/local/bin` will point to the correct places isn't a valid assumption. I bit myself with this locally when cooking up some magic. I think specifying the full path is the right thing to do.

/cc @schisamo @yzl
I'd love to see this branch integrated, too. Making the full path to chef configurable would enable me to use omnibus chef 11 in /opt/chef on an image containing an already installed chef 10 under /usr/local/bin.