$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