examples/distributor: new sample app

examples/distributor: new sample app

A new sample app that shows the usage of the distributor library. This
app works as follows:

* An RX thread runs which pulls packets from each ethernet port in turn
  and passes those packets to worker using a distributor component.
* The workers take the packets in turn, and determine the output port
  for those packets using basic l2forwarding doing an xor on the source
  port id.
* The RX thread takes the returned packets from the workers and enqueue
  those packets into an rte_ring structure.
* A TX thread pulls the packets off the rte_ring structure and then
  sends each packet out the output port specified previously by the worker
* Command-line option support provided only for portmask.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Signed-off-by: Reshma Pattan <reshma.pattan@intel.com>
Acked-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>