added ability to pass eval[1] into groups - [1]http://linux.die.net/man/...

I've needed to be able to pass an dhcpd-eval into the parameters portion of the group to do conditional PXE. Since the existing Parameter option automatically added ";" eval does not needed ";" added. This is a quick way to build a complex evaluation code and pass into the Array as Strings.

Example is below.

ipxe_eval = <<EOF
  if exists user-class and option user-class = "iPXE" {
    filename "bootstrap.ipxe";
  } else {
    filename "undionly.kpxe";
  }
EOF

group = data_bag_item("dhcp_groups","sj-servers")
dhcp_group group["id"] do
	# point to the current tftp server
	parameters [ "default-lease-time 86400", "next-server #{node[:dhcp][:next_server]}"]
	evals [ipxe_eval]
	hosts host_data
end