test_network_ip_availability: Skip IPv6 tests when configured so
test_network_ip_availability: Skip IPv6 tests when configured so

Use a class attribute known by the base class so that IPv6 tests
are skipped appropriately when CONF.network_feature_enabled.ipv6
is False.

Closes-Bug: #1561857
Change-Id: I93f76b7f7cd94ff484d2e4507500af97578ac71a
(cherry picked from commit 61a5bcb52e23fa39839c30cb4f13f8a26e115516)

bigswitch/neutron,huntxu/neutron,mahak/neutron,mahak/neutron,huntxu/neutron,igor-toga/local-snat,noironetworks/neutron,bigswitch/neutron,openstack/neutron,mahak/neutron,noironetworks/neutron,cloudbase/neutron,sebrandon1/neutron,sebrandon1/neutron,eayunstack/neutron,eayunstack/neutron,cloudbase/neutron,igor-toga/local-snat,openstack/neutron,openstack/neutron