srm: Use srm.enable.space-reservation rather than dcache.enable.space-reservation

Target: trunk
Request: 2.9
Request: 2.8
Request: 2.7
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/7009/