spacemanager: Move dcache.enable.space-reservation to proper file

It was originally declared in spacemanager.properties because it inherited
from a now deprecated space manager property. Since that's no longer the
case, we may just as well move it to the "normal" place.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Paul Milar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7576/