Add support OFP_PORT_MOD

[shun159]

port_downさせるとき

Pio::PortMod.new(
  port_no: 1,
  mac_address: '11:11:11:11:11:11',
  config: [:port_down],
  mask: [:port_down]
)

port_upさせるとき

Pio::PortMod.new(
  port_no: 1,
  mac_address: '11:11:11:11:11:11',
  config: [],
  mask: [:port_down]
)

[houndci-bot]

Missing frozen string literal comment.

[coveralls]

Coverage increased (+0.005%) to 97.542% when pulling 433cc96 on shun159:feature/ofp_port_mod into a64e9a0 on trema:develop.