heat_support_soft_anti_affinity

$ grep 'anti-affinity' -C3 /usr/lib/python2.7/site-packages/heat/engine/resources/openstack/nova/server_group.py
        POLICIES: properties.Schema(
            properties.Schema.LIST,
            _('A list of string policies to apply. '
              'Defaults to anti-affinity.'),
            default=['anti-affinity'],
            constraints=[
                constraints.AllowedValues(["anti-affinity", "affinity"]) <--- add here "soft-anti-affinity"
            ],
            schema=properties.Schema(
                properties.Schema.STRING,