This is not very clear.
just editing /home/stack/undercloud.conf is not enough,
We should write something like :
all ironic nodes using pxe_ssh needs to be migrated to VBMC.
Without which the minor upgrade fails.
Below statement does not clarify that migrating to VBMC is mandatory.
For more information about this new driver and migration instructions, see the Appendix "Virtual Baseboard Management Controller (VBMC)" in the Director Installation and Usage Guide.
Reported by: rhn-support-sbandyop