Use default for external_gateway_ip in multiple_nics_ipv6.j2

Use 'fe80::1' as the default external gateway for ipv6.
We earlier had an ipv4 address as the default in nic config
templates.

Closes-Bug: #1899627
Change-Id: Ia9bd8e17134fbdb83ab3fa49d21e0168ec33623a

tripleo_ansible/roles/tripleo_network_config/templates/ci/multiple_nics_ipv6.j2

@@ -15,7 +15,7 @@ network_config:
   routes:
   - default: true
     ip_netmask: ::/0
-    next_hop: {{ external_gateway_ip }}
+    next_hop: {{ external_gateway_ip | default('fe80::1') }}
   members:
   - type: interface
     name: nic2