Description of problem:
convert2rhel fails to download the "subscription-manager-initial-setup-addon" package and then refuses to continue. The said package is not available anymore in the CentOS repository.

subscription-manager-initial-setup-addon-1.25.17-1.el8.x86_64 Wed Feb 26 11:57:17 2020 1582685837 CentOS x86-01.mbox.centos.org RSA/SHA256, Thu Dec 5 12:47:31 2019, Key ID 05b555b38483c65d

Version-Release number of selected component (if applicable):
convert2rhel-0.25-1.el8

How reproducible:
Always

Steps to Reproduce:
1. Install CentOS 8.5
2. Install the "subscription-manager-initial-setup-addon" package from https://access.redhat.com/downloads/content/subscription-manager-initial-setup-addon/1.28.21-3.el8/x86_64/fd431d51/package
3. Convert to RHEL 8.5

Actual results:
convert2rhel itself tries to uninstall it as per this output:
~~~
Upon continuing, we will uninstall the following subscription-manager/katello-ca-consumer packages:

Package Vendor/Packager Repository
------- --------------- ----------
subscription-manager-initial-setup-addon-1.25.17-1.el8.x86_64 CentOS @AppStream
:
:
No package subscription-manager-initial-setup-addon available.
Exiting due to strict setting.
Error: No package subscription-manager-initial-setup-addon available.
:
:
CRITICAL - Couldn't download the subscription-manager-initial-setup-addon package. This means we will not be able to do a complete rollback and may put the system in a broken state.
Check to make sure that the CentOS Linux repositories are enabled and the package is updated to its latest version.
If you would rather ignore this check set the environment variable 'CONVERT2RHEL_UNSUPPORTED_INCOMPLETE_ROLLBACK'.
Traceback (most recent call last):
File "/usr/lib/python3.6/site-packages/convert2rhel/main.py", line 102, in main
pre_ponr_conversion()
File "/usr/lib/python3.6/site-packages/convert2rhel/main.py", line 193, in pre_ponr_conversion
subscription.replace_subscription_manager()
File "/usr/lib/python3.6/site-packages/convert2rhel/subscription.py", line 203, in replace_subscription_manager
remove_original_subscription_manager()
File "/usr/lib/python3.6/site-packages/convert2rhel/subscription.py", line 222, in remove_original_subscription_manager
utils.remove_pkgs(submgr_pkg_names, critical=False)
File "/usr/lib/python3.6/site-packages/convert2rhel/utils.py", line 369, in remove_pkgs
changed_pkgs_control.backup_and_track_removed_pkg(nvra)
File "/usr/lib/python3.6/site-packages/convert2rhel/utils.py", line 316, in backup_and_track_removed_pkg
restorable_pkg.backup()
File "/usr/lib/python3.6/site-packages/convert2rhel/utils.py", line 581, in backup
self.path = download_pkg(self.name, dest=BACKUP_DIR, set_releasever=False)
File "/usr/lib/python3.6/site-packages/convert2rhel/utils.py", line 483, in download_pkg
" 'CONVERT2RHEL_UNSUPPORTED_INCOMPLETE_ROLLBACK'." % (pkg, system_info.name)
File "/usr/lib/python3.6/site-packages/convert2rhel/logger.py", line 146, in _critical
sys.exit(msg)
SystemExit: Couldn't download the subscription-manager-initial-setup-addon package. This means we will not be able to do a complete rollback and may put the system in a broken state.
Check to make sure that the CentOS Linux repositories are enabled and the package is updated to its latest version.
If you would rather ignore this check set the environment variable 'CONVERT2RHEL_UNSUPPORTED_INCOMPLETE_ROLLBACK'.

WARNING - Abnormal exit! Performing rollback ...
~~~

Additional info:

• Uninstalling this package allows to continue the process.