There are cases when cinder_volume_node_names is not defined. It was
fixed for "set cinder_volume upgrade node facts from the limit option"
task in loop but never for non HA task. So, standalone upgrade job
fails with the following message.
"msg": "The task includes an option with an undefined variable. The
error was: 'cinder_volume_node_names' is undefined\n\nThe error appears
to be in
line 27, 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 cinder_volume upgrade node facts in a single-node
environment\n ^ here\n",
This patch defaults cinder_volume_node_names to  when
cinder_volume_node_names is not defined in hiera.