packaging: conf: re-add retry to apache proxy

removed at 734ba5516ae8 with no apparent reason.

Related-To: https://bugzilla.redhat.com/show_bug.cgi?id=961677
Change-Id: Ic43c8f10a38c5606ad59a92f05f7354ee6cd6287
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>