Add port option for an Ipv6 server fails

The add port option for a server when used on an ipv6-instance fails.

The server gets an ip automatically although no-dhcp is enabled while configuring subnet.Then many a time the server gets both the ip's.

 To Reproduce:

a.Create Network
network create net2 --provider-network-type vxlan
b. Create subnet
subnet create --ip-version=6 --no-dhcp --network net6 --subnet-range 2006:db9:cafe:e::/64 sub6

c. Create port with ipv6-address
port create --fixed-ip subnet=sub6,ip-address=2006:db9:cafe:e::6 --network net6 port1

d. Create Server:
server create --image cirros --flavor cirros --nic net-id=net6 vm1

server add port vm1 port1