inventory: Add system_images_registry example

system_images_registry is required for system container installations
but no example was put in the inventory files. This change adds a note
saying the variable is required when using system containers and
provides an example.

Ref: https://github.com/openshift/openshift-ansible/pull/5024

inventory/byo/hosts.origin.example

@@ -56,6 +56,9 @@ openshift_release=v3.6
 #openshift_use_node_system_container=False
 #openshift_use_master_system_container=False
 #openshift_use_etcd_system_container=False
+#
+# In either case, system_images_registry must be specified to be able to find the system images
+#system_images_registry="docker.io"
 
 # Install the openshift examples
 #openshift_install_examples=true

inventory/byo/hosts.ose.example

@@ -56,6 +56,9 @@ openshift_release=v3.6
 #openshift_use_node_system_container=False
 #openshift_use_master_system_container=False
 #openshift_use_etcd_system_container=False
+#
+# In either case, system_images_registry must be specified to be able to find the system images
+#system_images_registry="registry.access.redhat.com"
 
 # Install the openshift examples
 #openshift_install_examples=true