transfermanagers: Fix multi-domain issue

Fixes an inssue that prevented transfermanagers from being used in any
other domain that the srm services. This issue has apparently existed for
years, possibly from the very first version of the SRM.

Target: trunk
Require-notes: yes
Require-book: no
Acked-by: Albert Rossi <arossi@fnal.gov>
Patch: http://rb.dcache.org/r/6846/