Random public port range should be split in two attributes: star & end

[miguelaferreira]

This way we could better integrate with other tools (kitchen e.g.)

[bheuvel]

When using the latest PR created at kitchen-vagrant test-kitchen/kitchen-vagrant#167 ;

    pf_public_port_randomrange:
      start: 49160
      end: 49200

resulting/requested Vagrantfile:

pf_public_port_randomrange = {:start=>49160, :end=>49200}