Configure persistent shared Kubespray download cache path

[younius]

Summary

Define a repository-local, Git-ignored download cache path consistently for the eden, sol, and luna inventories without changing the default node-direct download mode.

Acceptance criteria

• Actual and example inventories use the same cache path under the Kubespray checkout.
• local_release_dir remains /tmp/releases as managed-node staging.
• README guidance distinguishes download_cache_dir from local_release_dir.
• download_run_once/download_localhost remain explicit opt-in because localhost image caching requires a compatible container runtime and privilege.

[younius]

Opened against the upstream repository by mistake; this change belongs to the eden-universe fork.