Fix Pools Total Disk Free graph to display bytes rather than kB

Ceph returns disk amounts in kB (i.e. divided by 1024).  Scale back
up by adding the graphite function scale(..,1024) to the template.

Fixes: #8853
Signed-off-by: John Spray <john.spray@inktank.com>
Reviewed-by: Dan Mick <dan.mick@inktank.com>