Create private IPv6 subnet specifying mode flags
Create private IPv6 subnet specifying mode flags

$ipv6_modes should always be passed when creating the
default IPv6 subnet, not just when fixed_range_v6 is
set.  Without it the default was DHCPv6, which cirros
doesn't support out of the box.  Was broken in
change-over from neutron to openstack cli.

Change-Id: Iadd39b1ce02fe0b3781bd3ae04adfd20d7e12d9f
Closes-bug: #1656098

openstack-dev/devstack,noironetworks/devstack,dtroyer/devstack,group-policy/devstack,openstack-dev/devstack,richliu/devstack,nawawi/openstack,dtroyer/devstack,nawawi/openstack,noironetworks/devstack,group-policy/devstack,richliu/devstack