Allow domain-name-servers for ipv6
Allow domain-name-servers for ipv6

Previously domain-name-servers option generation was turned off for ipv6
with generating other dhcp options ('routers' and
'dhcp-server-identifier').
Using this options caused failures for ipv6 on network creation.

But deeper investigation shows that 'domain-name-servers' is a valid
option for ipv6, so enabling it.

Closes: #64

arnaudcoquelet/infoblox-client,infobloxopen/infoblox-client