Adding the "special" option to dhcp::pool

  There are just way to may things you can do as part of a DHCP ip
  address pool definition to model them all, i.e. if statements.  This
  commit will add a parameter named "special" that you can use to inject
  unsanitized data into a pool definition...yes this means you need to
  know what you're doing and the format of a pool definition.

  Before this we couldn't easily support custom definitions needed be
  things like razor's iPXE.