ovs: Add port setting options

[chenyunchen]

ovs-vsctl --format=json set port bond0 vlan_mode=trunk trunk=1,2,3,4,5
ovs-vsctl --format=json get port bond0 tag vlan_mode trunk

[]          // If the option is not exist, OVS will return "[]\n"
trunk       // If the option is not exist, OVS will return "[]\n"
[1,2,3,4,5] // If the option is not exist, OVS will return "[]\n"

Only support the tag, vlan_mode, trunk option at this point.
All fields in PortOptions structs are pointers.

type PortOptions struct {
	Tag      *int
	VLANMode *string
	Trunk    []int
}
...
// Tag will be 0 if we just use init without pointer
err := c.VSwitch.Set.Port(port, PortOptions{
	VLANMode: vlanMode,
	Trunk:    trunk,
})

Or having an init function to create PortOptions. Any idea?