[CI] TestVolumeBootPattern.test_volume_boot_pattern failed because test file wasn't found

[Alexey Stupnikov]

nova-ceph-multistore failed for one of my patches because TestVolumeBootPattern.test_volume_boot_pattern failed to find test file.

test_volume_boot_pattern() [1] has quite simple logic:

- Create in Cinder some bootable volume importing a Glance image
- Boot an instance from the bootable volume
- Write content to the volume
- Delete an instance and Boot a new instance from the volume
- Check written content in the instance # FAILED
- Create a volume snapshot while the instance is running
- Boot an additional instance from the new snapshot based volume
- Check written content in the instance booted from snapshot

For some reason test file in /tmp folder was missing after new instance was created with this volume attached. I have no idea what may cause it other than some cleanup operation against /tmp folder on OS level. Reporting this bug just in case, maybe there is some obvious problem.

Logs: https://zuul.opendev.org/t/openstack/build/42a6c06ff5f545a5bb610d7c0d454032/logs

[1]
https://github.com/openstack/tempest/blob/d1742bad94e94ebabe875ed24415d1ff510a5ae7/tempest/scenario/test_volume_boot_pattern.py#L58