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
3 files changed