scylla_create_devices fails when var-lib-scylla.mount already exists

[ShlomiBalalis]

During a restart_then_repair nemesis in SCT, we've noticed that when we run scylla_create_devices on a restarted scylla instance the script fails when var-lib-scylla.mount already exists, a behavior we've never seen beforehand, and it is not a part of the known procedure.

As for SCT, at the moment we have a temporary fix for the issue: scylladb/scylla-cluster-tests#2839 which we will remove once this issue is fixed

[yaronkaikov]

@fruch ^

[fruch]

@fruch ^

We need to handle this inside create_devices scripts.
otherwise it would be break any user that would be trying to follow the procedure:
https://docs.scylladb.com/operating-scylla/procedures/cluster-management/rebuild-node/

if one would follow this procedure, he would fail can calling scylla_create_devices after restart of a node