Set up temporary (single-host) ceph pool

By default, Ceph uses host as failure domain (which means it forces placement groups to replicate across multiple hosts).

Our temporary setup has a single host (until the hypervisor3 ssds are emptied and repurposed). We need to set up a temporary CRUSH rule allowing replication on a single host.

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Migrated from T2507 (view on Phabricator)