Be less confusing about custom Docker registry

Docker registry password is, by default, sourced from
passwords.yml file.

Cleans up globals.yml to make it clearer.
Also follows the "present defaults" behaviour of the other vars.

Change-Id: Icc993e82a6a435f948e3d17e410eb14717cb0e2d

etc/kolla/globals.yml

@@ -55,14 +55,16 @@
 ################
 # Docker options
 ################
-# Below is an example of a private repository with authentication. Note the
-# Docker registry password can also be set in the passwords.yml file.
-
-#docker_registry: "172.16.0.10:4000"
-#docker_namespace: "companyname"
-#docker_registry_insecure: "yes"
-#docker_registry_username: "sam"
-#docker_registry_password: "correcthorsebatterystaple"
+
+# Custom docker registry settings:
+#docker_registry:
+#docker_registry_insecure: "{{ 'yes' if docker_registry else 'no' }}"
+#docker_registry_username:
+# docker_registry_password is set in the passwords.yml file.
+
+# Namespace of images:
+#docker_namespace: "kolla"
+
 # Docker client timeout in seconds.
 #docker_client_timeout: 120