Fixing logic for cpu_model parameter
Backporting from pike to ocata and newton
Adding docs and tests

Prod-Related: PROD-19272

Change-Id: I8948f48e4ef57ece3ccdee76a5152536cced5a3c
diff --git a/README.rst b/README.rst
index 3abd1fb..e0ba486 100644
--- a/README.rst
+++ b/README.rst
@@ -633,6 +633,18 @@
     compute:
       cpu_mode: host-model
 
+Nova compute cpu model
+----------------------
+
+.. code-block:: yaml
+
+  nova:
+    compute:
+      cpu_mode: custom
+      libvirt:
+        cpu_model: IvyBridge
+
+
 Nova compute workarounds
 ------------------------