As described in the related bug we need to use dnf instead of yum for centos8 otherwise it fails in the undercloud packages update. I am using the python version to determine - py3 is dnf. As part of discussion here when dnf isn't available fall back to yum. Found as part of the work in [1]. Related-Bug: 1886837 [1] https://tree.taiga.io/project/tripleo-ci-board/task/1817 Change-Id: Idac62d37a19ee49f30936e20021a9dab5af40eec