Blueprint image-precache-support added a conf section called
[image_cache], so it makes sense to move all the existing image
cache-related conf options into it.
Old:
[DEFAULT]image_cache_manager_interval
[DEFAULT]image_cache_subdirectory_name
[DEFAULT]remove_unused_base_images
[DEFAULT]remove_unused_original_minimum_age_seconds
[libvirt]remove_unused_resized_minimum_age_seconds
New:
[image_cache]manager_interval
[image_cache]subdirectory_name
[image_cache]remove_unused_base_images
[image_cache]remove_unused_original_minimum_age_seconds
[image_cache]remove_unused_resized_minimum_age_seconds
Change-Id: I3c49825ac0d70152b6c8ee4c8ca01546265f4b80
Partial-Bug: #1847302
828e8047e5