Allow customizing node upgrade serial value.

playbooks/common/openshift-cluster/upgrades/upgrade_nodes.yml

@@ -1,7 +1,9 @@
 ---
 - name: Evacuate and upgrade nodes
   hosts: oo_nodes_to_config
-  serial: 1
+  # This var must be set with -e on invocation, as it is not a per-host inventory var
+  # and is evaluated early. Values such as "20%" can also be used.
+  serial: "{{ openshift_upgrade_nodes_serial | default(1) }}"
   any_errors_fatal: true
   roles:
   - openshift_facts