Disable ipv6

- When starting mininet with arbitrary topology by using ryu as a remote controller, there are plenty of packets sends from each hosts to a set of strange mac addresses start with 33:33:XX:XX...

- I didn't send these pkts (they started sending immediately after mininet network was set up), and I didn't want they show up. These irritating messages kept refreshing the controller console and led to debug trouble.

- By https://en.wikipedia.org/wiki/IP_multicast and http://www.iana.org/assignments/ethernet-numbers/ethernet-numbers.xhtml, knowing that mac addresses start with 33:33:XX:XX is a part of ipv6 multicast addresses. So disable ipv6 in mininet is what I want to do.

- According to https://github.com/mininet/mininet/wiki/Mininet-2.2.0-Release-Notes, "IPv6 keeps coming back, even though we try to disable it!" 

- And also by previous link and https://github.com/mininet/mininet/issues/454, modify the value of "GRUB_CMDLINE_LINUX_DEFAULT" as "ipv6.disable=1 text" in /etc/default/grub will disable ipv6 in whole virtual machine (not only in mininet).

- after upgrading grub, the irritating packets disappear!