Make image_cache_manager_interval configurable

Add opportunity to change image_cache_manager_interval in reclass
model. Now it hardcoded to 0 in nova-compute config template that
cause high cpu load issues on environments with nfs storage enabled.

Related-PROD: PROD-36930
Change-Id: I2229d5681a7ed183d2af3cf87526f7c378421228
diff --git a/README.rst b/README.rst
index 2db01f1..fa1253f 100644
--- a/README.rst
+++ b/README.rst
@@ -601,6 +601,7 @@
     nova:
       compute:
         instances_path: /mnt/nova/instances
+        image_cache_manager_interval: 2400
 
     linux:
       storage:
@@ -613,6 +614,8 @@
             file_system: nfs
             opts: rw,vers=3
 
+.. note:: Parameter 'image_cache_manager_interval' recommended to set to 2400 for the NFS shared storage setup (PROD-36930)
+
 Nova flavors:
 
 .. code-block:: yaml