Currently, there is no dns servers prioritization for subnets
for Neutron.
Generally speaking, it is useful to keep the order of dns
nameservers consistent. Add a new column named 'order' in table
'dnsnameservers' and add nameserver into DB one by one.
Closes-Bug: #1218629
Implements: blueprint keep-dns-nameserver-orderconsistency
Change-Id: Id937aea411397d39370368a4eb45be26c4eefa9e
cb60d0bb4e