Fix neutron ipv6 doc
Fix neutron ipv6 doc

ipv6_pd_enabled is supported by later verion of neutron.

Change-Id: I2f9d1ff709e5efb44218bf7199884a3438cc9af5

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals