Ensure public net id is configured for Kuryr SDN

This PR ensures the playbook execution will fail soon, at the
prerequisites checkings, if the public net id is not configured when
the Kuryr SDN is used.

roles/openshift_openstack/tasks/net_vars_check.yaml

@@ -12,3 +12,10 @@
   when:
     - openstack_private_data_network_name is defined
     - not openshift_use_flannel|default(False)|bool
+
+- name: Check the kuryr public net configuration
+  fail:
+    msg: "kuryr_openstack_public_net_id must be set when using Kuryr SDN"
+  when:
+    - openshift_use_kuryr|default(False)|bool
+    - not kuryr_openstack_public_net_id is defined