During development of jira #OSP-25480 issue was hit which prevents proper upgrade of cell nodes:

2023-06-07 22:53:18 | 2023-06-07 22:53:18.879431 | 525400ce-df83-5ec9-0a2b-00000000047f |      FATAL | set mysql upgrade node facts in a single-node environment | cell1-cellcontrol-0 | error={"msg": "The task includes an option with an undefined variable. The error was: 'mysql_node_names' is undefined\n\nThe error appears to be in '/home/stack/overcloud-deploy/cell1/config-download/cell1/CellController/upgrade_tasks_step1.yaml': line 177, column 5, but may\nbe elsewhere in the file depending on the exact syntax problem.\n\nThe offending line appears to be:\n\n- block:\n  - name: set mysql upgrade node facts in a single-node environment\n    ^ here\n"}