Add 'manage_service_maintenance'. Don't manage services scheduling while upgrading.

For some special cases, don't manage services scheduling both enable and
disable before and after upgrade procedure.

Change-Id: I2499e5240cfc8b1ddcf91948fa052590c6a161d1
Related-Prod: PROD-30156
diff --git a/README.rst b/README.rst
index 0f6117d..b1fa824 100644
--- a/README.rst
+++ b/README.rst
@@ -1424,3 +1424,20 @@
 |                               | operations, verify do not have dead network          |
 |                               | agents/compute services)                             |
 +-------------------------------+------------------------------------------------------+
+
+
+Don't manage services scheduling while upgrade
+----------------------------------------------
+For some special cases, don't manage services scheduling both enable and disable
+before and after upgrade procedure.
+
+If 'manage_service_maintenance: true' or not present - default behavior, disable services
+before upgrade and enable it after upgrade.
+If 'manage_service_maintenance: false' - don't disable and don't enable upgraded services
+scheduling before and after upgrade.
+
+.. code-block:: yaml
+
+    nova:
+      upgrade:
+        manage_service_maintenance: false