test tool: run_mininet: enable to assign the different types of the switches

this patch makes run_mininet.py possible to:

  - select the differenct types of the switches
  - support LINC Switch when the mininet was got from
    https://github.com/FlowForwarding/mininet

usage)

    sudo ryu/tests/switch/run_mininet.py --tester ovs --target linc