Fix job queue directory permission problems

If split users are used, the queue directory could only be accessed
by masterd, but also confd needs to be able to read it, e.g. when it
is queried as part of "gnt-job list"

This commit fixes the permissions in such a way to allow proper access rights.

Fixes Issue 406.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>