Kolla recently upgraded bifrost from 7.0.0 to 7.1.0 on the stable/train
branch. This switched to IPA builder to build the IPA image, and
introduced a rename of the IPA kernel file from ipa.vmlinuz to
ipa.kernel, which breaks overcloud provisioning. The iPXE kernel
download fails with a 404, since Kayobe introspection rules use
ipa.vmlinuz for the driver_info.deploy_kernel URL.

This change works around the issue by setting two Bifrost variables,
ipa_kernel and ipa_kernel_url, to reference the old kernel filename of
ipa.vmlinuz. This works both in the case where the image is downloaded
from a URL (ipa_kernel sets the destination file name), and where it is
built via 'kayobe seed deployment image build' (kayobe uses the legacy
ironic-agent DIB element rather than IPA builder, which creates a
hardlink to ipa.vmlinuz).

We chose the above approach rather than switching to IPA builder due to
it being a less risky change at a time close to release. A future
release of Kayobe should switch to IPA builder, but this will be a
larger effort.

[1] https://review.opendev.org/#/c/692200/1/playbooks/roles/bifrost-ironic-install/defaults/main.yml

Change-Id: I7f75c25602fd7ae4bfeb6abbdd3b42d8ee465abf
Story: 2007068
Task: 37951