system-test: Fix authzdb generation when htpasswd is not installed

Otherwise gPlazma will fail due to the missing configuration file.

Target: trunk
Request: 2.7
Require-notes: no
Require-book: no
Acked-by: Karsten Schwank <karsten.schwank@desy.de>
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/5927/