Nova VM creation fails with "Permission denied" errors after a minor update

Issue

• After a RHOSP 12 or 13 minor update, VM creation started failing with "Permission denied" errors :

2019-05-04 12:58:57.676 1 ERROR nova.compute.manager [req-07f73414-0cf4-4ab5-aa51-191628dc3be2 1619ba64849e406eb622d4d064d1748e d98e6bde8f884662bcc7af95b34b9057 - def
ault default] [instance: 0b360997-a148-470d-8f89-00a80a214704] Instance failed to spawn: OSError: [Errno 13] Permission denied: '/var/lib/nova/instances/0b360997-a148
-470d-8f89-00a80a214704'
2019-05-04 12:58:57.676 1 ERROR nova.compute.manager [instance: 0b360997-a148-470d-8f89-00a80a214704] Traceback (most recent call last):
2019-05-04 12:58:57.676 1 ERROR nova.compute.manager [instance: 0b360997-a148-470d-8f89-00a80a214704]   File "/usr/lib/python2.7/site-packages/nova/compute/manager.py
", line 2188, in _build_resources
2019-05-04 12:58:57.676 1 ERROR nova.compute.manager [instance: 0b360997-a148-470d-8f89-00a80a214704]     yield resources
2019-05-04 12:58:57.676 1 ERROR nova.compute.manager [instance: 0b360997-a148-470d-8f89-00a80a214704]   File "/usr/lib/python2.7/site-packages/nova/compute/manager.py
", line 2003, in _build_and_run_instance
2019-05-04 12:58:57.676 1 ERROR nova.compute.manager [instance: 0b360997-a148-470d-8f89-00a80a214704]     block_device_info=block_device_info)
2019-05-04 12:58:57.676 1 ERROR nova.compute.manager [instance: 0b360997-a148-470d-8f89-00a80a214704]   File "/usr/lib/python2.7/site-packages/nova/virt/libvirt/drive
r.py", line 2889, in spawn
2019-05-04 12:58:57.676 1 ERROR nova.compute.manager [instance: 0b360997-a148-470d-8f89-00a80a214704]     block_device_info=block_device_info)
2019-05-04 12:58:57.676 1 ERROR nova.compute.manager [instance: 0b360997-a148-470d-8f89-00a80a214704]   File "/usr/lib/python2.7/site-packages/nova/virt/libvirt/drive
r.py", line 3266, in _create_image
2019-05-04 12:58:57.676 1 ERROR nova.compute.manager [instance: 0b360997-a148-470d-8f89-00a80a214704]     fileutils.ensure_tree(libvirt_utils.get_instance_path(instan
ce))
2019-05-04 12:58:57.676 1 ERROR nova.compute.manager [instance: 0b360997-a148-470d-8f89-00a80a214704]   File "/usr/lib/python2.7/site-packages/oslo_utils/fileutils.py
", line 40, in ensure_tree
2019-05-04 12:58:57.676 1 ERROR nova.compute.manager [instance: 0b360997-a148-470d-8f89-00a80a214704]     os.makedirs(path, mode)
2019-05-04 12:58:57.676 1 ERROR nova.compute.manager [instance: 0b360997-a148-470d-8f89-00a80a214704]   File "/usr/lib64/python2.7/os.py", line 157, in makedirs
2019-05-04 12:58:57.676 1 ERROR nova.compute.manager [instance: 0b360997-a148-470d-8f89-00a80a214704]     mkdir(name, mode)
2019-05-04 12:58:57.676 1 ERROR nova.compute.manager [instance: 0b360997-a148-470d-8f89-00a80a214704] OSError: [Errno 13] Permission denied: '/var/lib/nova/instances/
0b360997-a148-470d-8f89-00a80a214704'