httpd: Parameterize war unpack directory

Addresses the issue that the webapp unpack directory is /var/tmp in
tar and system-test builds. For these builds the unpack directory
should be within the var directory of the installation directory.

Target: trunk
Request: 2.6
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/5753/