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 old names, and use if present; otherwise use the new names.

Fixes: #10127
Signed-off-by: Dan Mick <dan.mick@redhat.com>
This is the wrong order to check. Consider the situation where a cluster is upgraded from firefly to giant. In this case it would be possible for graphite to contain both stats. If you have this in place it seems like you'd be serving stale data for a week. Check if the new data is available first.