Support custom nameservers in setup-neutron

Allow the network description to pass in the nameserver to be used
for the subnet, falling back to the default of 8.8.8.8 for floating
networks if one is not specified.

Change-Id: Ibd8a33b0f1a3e26fb14b5050dd7e536828a3f3eb
Merge "Support custom nameservers in setup-neutron"