Avoid shared IPA image cache on Ansible control host

When using overcloud Ironic, IPA images for Ironic inspector are
downloaded to the Ansible control host to a cache directory, by default
/opt/kayobe/images/ipa/. They are later copied into the local Kolla
Ansible configuration under
etc/kolla/config/ironic/ironic-agent.{initramfs,kernel}.

The use of a shared cache directory results in problems when multiple
users share a single Ansible control host, since the cache is created as
writeable only for the user that created it. Other users sharing the
same Ansible control host will be unable to write to the cache.

We may also see issues if multiple Kayobe environments using different
IPA images are deployed from one Ansible control host.

The cache is not strictly necessary, since we can download the images
directly to the kayobe-config repo. This change avoids the use of the
cache. The performance impact should be minimal, only requiring an
additional download when a fresh kayobe-config is used.

Change-Id: I022c53afc0f64ccc79eeff4a220ade4c9216edfc
Closes-Bug: #2069845