Fix use of IPA images in Bifrost without a checksum

Ironic now provides DIB-built CentOS images at
https://tarballs.openstack.org/ironic-python-agent/dib/files. However,
if Bifrost is configured to use these, then the 'kayobe seed service
deploy' command fails because the checksum files are not available.

This change fixes the issue by allowing
'kolla_bifrost_ipa_kernel_checksum_url' and
'kolla_bifrost_ipa_ramdisk_checksum_url' to be overridden to empty
strings, indicating that there is no checksum.

Very similar to the issue fixed in https://review.opendev.org/688359 for
the overcloud.

Change-Id: If73cebb95c7d0596dc8087112fc19398a113f0cc
Story: 2006715
Task: 37407

ansible/roles/kolla-bifrost/templates/bifrost.yml.j2

@@ -40,35 +40,23 @@ inspector_extra_kernel_options: "{{ kolla_bifrost_inspector_extra_kernel_options
 # Whether to download Ironic Python Agent (IPA) images.
 download_ipa: "{{ kolla_bifrost_download_ipa }}"
 
-{% if kolla_bifrost_ipa_kernel_upstream_url %}
 # URL of Ironic Python Agent (IPA) kernel image.
 ipa_kernel_upstream_url: "{{ kolla_bifrost_ipa_kernel_upstream_url }}"
-{% endif %}
 
-{% if kolla_bifrost_ipa_kernel_checksum_url %}
 # URL of checksum of Ironic Python Agent (IPA) kernel image.
 ipa_kernel_upstream_checksum_url: "{{ kolla_bifrost_ipa_kernel_checksum_url }}"
-{% endif %}
 
-{% if kolla_bifrost_ipa_kernel_checksum_algorithm %}
 # Algorithm of checksum of Ironic Python Agent (IPA) kernel image.
 ipa_kernel_upstream_checksum_algo: "{{ kolla_bifrost_ipa_kernel_checksum_algorithm }}"
-{% endif %}
 
-{% if kolla_bifrost_ipa_ramdisk_upstream_url %}
 # URL of Ironic Python Agent (IPA) ramdisk image.
 ipa_ramdisk_upstream_url: "{{ kolla_bifrost_ipa_ramdisk_upstream_url }}"
-{% endif %}
 
-{% if kolla_bifrost_ipa_ramdisk_checksum_url %}
 # URL of checksum of Ironic Python Agent (IPA) ramdisk image.
 ipa_ramdisk_upstream_checksum_url: "{{ kolla_bifrost_ipa_ramdisk_checksum_url }}"
-{% endif %}
 
-{% if kolla_bifrost_ipa_ramdisk_checksum_algorithm %}
 # Algorithm of checksum of Ironic Python Agent (IPA) ramdisk image.
 ipa_ramdisk_upstream_checksum_algo: "{{ kolla_bifrost_ipa_ramdisk_checksum_algorithm }}"
-{% endif %}
 
 {% if kolla_bifrost_extra_globals %}
 ###############################################################################