Update ndfc_underlay_ip_address.j2 (#546)

Author: ccoueffe

roles/dtc/common/templates/ndfc_underlay_ip_address.j2

@@ -53,7 +53,7 @@
 
 {% if vxlan.topology.fabric_links is defined %}
 {% for switch in vxlan.topology.fabric_links %}
-{% if switch.ipv4 is iterable or switch.ipv6 is iterable %}
+{% if switch.ipv4 is defined and switch.ipv4 is iterable %}
 - entity_name: "{{ switch_list[switch.source_device].serial_number }}~{{ switch.source_interface }}~{{ switch_list[switch.dest_device].serial_number }}~{{ switch.dest_interface }}"
   pool_type: SUBNET
   pool_name: "SUBNET"