Provide ability to configure keystone resync interval

Contrail API provides an option 'keystone_resync_interval_secs'.
Its default value of 60sec is not always convenient.
This commit introduces support of this param in formula.

Change-Id: I43bff19d4120fe86759efb56f28afc64e44e2d91
Related-PROD: PROD-27794
diff --git a/README.rst b/README.rst
index 44c9e7c..75ac251 100644
--- a/README.rst
+++ b/README.rst
@@ -897,6 +897,17 @@
           sync_on_demand: false
       ...
 
+Configure duration between polls to keystone to sync domains and projects
+-------------------------------------------------------------------------
+
+.. code-block:: yaml
+
+    opencontrail:
+      ...
+      config:
+        keystone_resync_interval_secs: 60
+      ...
+
 Cassandra listen configuration
 ------------------------------