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 15a48c0..ef95fe9 100644
--- a/README.rst
+++ b/README.rst
@@ -597,6 +597,7 @@
     nova:
       compute:
         instances_path: /mnt/nova/instances
+        image_cache_manager_interval: 2400
 
     linux:
       storage:
@@ -609,6 +610,8 @@
             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
diff --git a/nova/files/pike/nova-compute.conf.Debian b/nova/files/pike/nova-compute.conf.Debian
index 0e61bee..9b29a18 100644
--- a/nova/files/pike/nova-compute.conf.Debian
+++ b/nova/files/pike/nova-compute.conf.Debian
@@ -775,7 +775,7 @@
 #  (integer value)
 # Minimum value: -1
 #image_cache_manager_interval=2400
-image_cache_manager_interval=0
+image_cache_manager_interval={{ compute.get('image_cache_manager_interval', '0') }}
 
 #
 # Interval to pull network bandwidth usage info.
diff --git a/nova/files/queens/nova-compute.conf.Debian b/nova/files/queens/nova-compute.conf.Debian
index cffddcf..cd04c5e 100644
--- a/nova/files/queens/nova-compute.conf.Debian
+++ b/nova/files/queens/nova-compute.conf.Debian
@@ -840,7 +840,7 @@
 # * Any other value
 #  (integer value)
 # Minimum value: -1
-image_cache_manager_interval = 0
+image_cache_manager_interval = {{ compute.get('image_cache_manager_interval', '0') }}
 
 #
 # Interval to pull network bandwidth usage info.