rest-api: fix Space get for ceph giant

From http://ceph.com/uncategorized/v0-87-giant-released/:
"Similarly, the total_space, total_used, and total_avail fields are
replaced with total_bytes, total_used_bytes, and total_avail_bytes
fields."

Update views/v1.py according to these changes, so that the main
dashboard can display cluster space usage.

Signed-off-by: Cédric Barboiron <ced@winkie.fr>