Make sure dnsmasq can distinguish IPv6 address from MAC address

Currrently, due to in dnsmasq host file, the IPv6 address does
not be wrapped with '[]', dnsmasq cannot distinguish IPv6 address
from MAC address, it will cause the deployed instances cannot get
IPv6 address via dnsmasq dhcp service.

Change-Id: I023e44667a238664d11f2ac2cc484432cd301bcc
Closes-Bug: #1281916
Merge "Make sure dnsmasq can distinguish IPv6 address from MAC address"