Since [1] nova-compute uses rbd python library instead of libvirt to cleanup
volumes and get pool info - so it requires cinder keyring on filesystem.

In external ceph case it is often that nova key does not exist (is simply a copied
cinder key) and the rbd user is set to cinder - therefore the earlier mentioned
operations will fail due to a missing keyring on the filesystem.

[1]: https://review.opendev.org/#/c/668564/

Change-Id: Idef21dc5f7e9ff512bc8920630a3de61a1e69eee
Backport: train
Closes-Bug: #1859408