Omit external VIP variables if not set

Change-Id: I324832f0e037be39e77e052ab515362906eee961
diff --git a/playbooks/roles/configure-installer/templates/kolla-ansible-extra-vars.j2 b/playbooks/roles/configure-installer/templates/kolla-ansible-extra-vars.j2
index a8fd195..2a9a7f1 100644
--- a/playbooks/roles/configure-installer/templates/kolla-ansible-extra-vars.j2
+++ b/playbooks/roles/configure-installer/templates/kolla-ansible-extra-vars.j2
@@ -21,8 +21,8 @@
 kolla_install_type: "{{ os_kolla_install_type }}"
 openstack_release: "{{ openstack_version }}"
 kolla_internal_vip_address: "{{ os_kolla_internal_vip_address }}"
-kolla_external_vip_address: "{{ os_kolla_external_vip_address }}"
-kolla_external_vip_interface: "{{ os_kolla_external_vip_interface }}"
+kolla_external_vip_address: "{{ os_kolla_external_vip_address | default(omit) }}"
+kolla_external_vip_interface: "{{ os_kolla_external_vip_interface | default(omit) }}"
 
 # TODO: The below options should be moved to scenario specifications
 # in infra/swconfig repository