Add missing download-dir-url entry

For now this entry is not used but it will be used in future versions of
slapcache. So make sure that from now on, the entry exists.
2 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-Master.Python2

00:12:08

passed SlapOS.Eggs.UnitTest-Master.Python3

00:12:35