keep dns nameserver order consistency

Currently, there is no dns servers prioritization for subnets
for Neutron.

Fox example:
command1: neutron subnet-update [subnet_id] --dns_nameservers list=true 1.1.1.1 2.2.2.2 3.3.3.3
command2: neutron subnet-update [subnet_id] --dns_nameservers list=true 3.3.3.3 2.2.2.2 1.1.1.1

Generally speaking, in Neutron, command1 is same as command2. These commands are update dns_nameservers of subnet. There is no priority about dns nameservers. But in many situations, there are many differences between every dns nameserver. So we want Neutron to keep dns nameserver order consistency.