rest-api/v1: adapt to new Ceph df output (total_bytes etc.)

Ceph changed the units (kB -> B) and name (total_space -> total_bytes,
etc.) with ee2dbdb0f5e54fe6f9c5999c032063b084424c4c in Firefly.  Look
for the new names, and use if present; otherwise use the old names.

Fixes: #10127
Signed-off-by: Dan Mick <dan.mick@redhat.com>