tox: Support DNS_INCLUDE_DIR and generated DNS

This change adds plumbing to tox and deploy-check.py to either
use a pre-arranged checkout of the new generated DNS entries from
netbox, or to create a new temporary one and insert it into the
generated zones tree.

Bug: T243362
Change-Id: Ia9eefdebf44d4c9ceb6302590453b79f0f3191c4