Make os-brick find the client key

As of some version between Zed and 2024.1, os_brick hardcodes the path to the Ceph credentials. In that path, it expects a file containing a ceph config snippet which contains the necessary configuration to find the key.

This code path of os_brick is not used in all operations related to cinder. We only came across this when running tempest test ID id-d8f1ca95-3d5b-44a3-b8ca-909691c9532d. Many other operations work fine.

Note that id-d8f1ca95-3d5b-44a3-b8ca-909691c9532d will still fail unless yaook/images/cinder!149 (merged) is also applied.