spacemanager: Fix use of global property in spacemanager configuration

Spacemanager did not respect the spacemanager.enable.space-reservation
property.

Target: trunk
Request: 2.7
Require-notes: yes
Require-book: no
Acked-by: Dmitry Litvintsev <litvinse@fnal.gov>
Patch: http://rb.dcache.org/r/6388/