Extend backup preparation test with an opportunity to create backup for Cloud cluster

[mikliapko]

We've recently added the separate backup preparation test (test_prepare_backup_snapshot) which creates the backup for their future usage in restore tests.

This helper test should be extended to support Cloud clusters as well for the purpose of 1-to-1 restore testing.

Scope:

• support Cloud clusters;
• address post-merge comments (Add a helper test to prepare Manager backup snapshots scylla-cluster-tests#9491) to the original PR