doc: need admin guide for the image cache

[Matt Riedemann]

There is no documentation for the image cache, so we should add one to the admin guide.

I think a relatively simple beginning would include:

- A high level description of what an image cache is, where it lives, and the benefits.

- Which compute drivers support image cache (that's not detailed here either: https://docs.openstack.org/nova/latest/user/support-matrix.html), this is any driver that supports the "has_imagecache" driver capability (currently libvirt, hyperv and vmware).

- The related configuration options since the options are not in a particular config option group they are all spread across DEFAULT (moving those to an [imagecache] group would probably be useful as well outside the docs change).

More advanced topics could be things like known issues/limitations (maybe mdbooth can help here), some of which is probably covered in this spec:

https://specs.openstack.org/openstack/nova-specs/specs/ussuri/approved/image-precache-support.html

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/690723

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/687348
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=829ccbe2bbc4e80e92baf7553401d7925d883732
Submitter: Zuul
Branch: master

commit 829ccbe2bbc4e80e92baf7553401d7925d883732
Author: Dan Smith <email address hidden>
Date: Tue Oct 8 09:15:12 2019 -0700

    Add image precaching docs for aggregates

    Related to blueprint image-precache-support

    Partial-Bug: #1847302
    Change-Id: I7a57e5e09b2a1760a9c5aeac402911895dfce07d

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/690748

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/690723
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=828e8047e5c8651ea757bda7922670889d5e8818
Submitter: Zuul
Branch: master

commit 828e8047e5c8651ea757bda7922670889d5e8818
Author: Eric Fried <email address hidden>
Date: Wed Oct 23 14:41:36 2019 -0500

    Consolidate [image_cache] conf options

    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

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/690748
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=f7c027db9abb847433df1dbe00d0e5fd9090c5c5
Submitter: Zuul
Branch: master

commit f7c027db9abb847433df1dbe00d0e5fd9090c5c5
Author: Eric Fried <email address hidden>
Date: Wed Oct 23 16:03:59 2019 -0500

    Add image caching to the support matrix

    Add a section to the support matrix for image caching
    (``has_imagecache`` virt driver capability).

    Change-Id: I9147c5ea6b276b4fe18a981f4360844009bd3d95
    Partial-Bug: #1847302