httpd: Use service specific property for list of login brokers

Part of the httpd service used dcache.service.loginbroker rather
than httpd.service.loginbroker. This patch resolves the issue.

The patch introduces httpd.setup to define the setup file of the
httpd service, thus making the reference to the file explicit in
the configuration.

Target: trunk
Request: 2.10
Require-notes: yes
Require-book: yes
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: https://rb.dcache.org/r/7266/
(cherry picked from commit 69de830289824943d7800931312fbecdd4923f05)
Merge pull request #825 from gbehrmann/fix/2.10/rb7266

httpd: Use service specific property for list of login brokers