replace internal with openstack_interface

replace harcode 'internal' with {{ openstack_interface }}

Change-Id: I885622967ffde2a7a1a08fedbde2eb0e4e330e22

ansible/roles/nova-cell/tasks/discover_computes.yml

@@ -27,7 +27,7 @@
   become: true
   command: >
     docker exec kolla_toolbox openstack
-    --os-interface internal
+    --os-interface {{ openstack_interface }}
     --os-auth-url {{ keystone_admin_url }}
     --os-identity-api-version 3
     --os-project-domain-name {{ openstack_auth.domain_name }}