LU-792 lbuild-rhel5 should use redhat's SRPM repo

LU-792 lbuild-rhel5 should use redhat's SRPM repo

Download EL5 kernel .src.rpm packages from redhat's repo instead
of trying to use CentOS's often-outdated repo.

Change-Id: I45c59894c649f3290a8ce1f0d3f1f033395567a4
Signed-off-by: Michael MacDonald <mjmac@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/1595
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Brian J. Murrell <brian@whamcloud.com>
Reviewed-by: Johann Lombardi <johann@whamcloud.com>