URL for packages and LiveOS differs from x86, x86_64 and ARM.

This patch allows to select the good mirror URL according to
the architecture.

Primary architecture: http://mirrors.kernel.org/fedora
Secondary architecture: http://mirrors.kernel.org/fedora-secondary

The managed secondary architectures are only ppc64 and s390x,
the secondary architectures for Fedora 20 (the base of initial bootstrap).
Signed-off-by: Laurent Vivier <Laurent@Vivier.EU>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>