[enterprise-3.7] Add openshift_docker_systemcontainer_image_override

(cherry picked from commit 6b18bbe) xref:openshift#6403

Author: adellape

install_config/install/advanced_install.adoc

@@ -679,7 +679,7 @@ from the default `registry.access.redhat.com/openshift3/`. To do so, set the
 following cluster variable in your inventory file in the `[OSEv3:vars]` section:
 
 ----
-openshift_docker_systemcontainer_image_registry_override="registry.example.com/myrepo/"
+openshift_docker_systemcontainer_image_override="<registry>/<user>/<image>:<tag>"
 ----
 
 [[advanced-install-etcd-system-container]]