Allow to use external nodegenerator

  * Switch to downstream reclass
  * Add reference replacement in generator:
   COLON_SYMBOL_FOR_FUTURE_REFERENCE => ':'

https://mirantis.jira.com/browse/PROD-30702

Change-Id: I342e2e3abc770f3a150668df06cc06fe0f3e2687
diff --git a/README.rst b/README.rst
index c871b99..c7959f5 100644
--- a/README.rst
+++ b/README.rst
@@ -204,7 +204,10 @@
 
 Multiple nodes definitions (using generator) with IP address comprehension.
 Ranges are named and formatting symbol of the same name is replaced by IP
-address from the corresponding range:
+address from the corresponding range.
+Some extra references supported, as:
+  COLON_SYMBOL_FOR_FUTURE_REFERENCE = ':'
+Which will be replaced during reclass process
 
 .. code-block:: yaml
 
@@ -236,6 +239,9 @@
                 tenant_address:
                   start: 101
                   value: 192.168.2.<<tenant_address>>
+                somevalue:
+                  start: 101
+                  value: 192.168.2.<<tenant_address>>COLON_SYMBOL_FOR_FUTURE_REFERENCEtestme
 
 Read more
 =========