use altarch mirror for CentOS on arches other than i386 and x86_64
use altarch mirror for CentOS on arches other than i386 and x86_64

CentOS only has i386 and x86_64 listed in mirrorlist.centos.org,
use http://mirror.centos.org/altarch/ for all other arches instead

See: https://bugs.centos.org/view.php?id=11250

Signed-off-by: Evgeni Golov <19053590b75b2e27abe838a7e75cee7f61ba3686@debian.org>

hallyn/lxc,federicobriata/lxc,federicobriata/lxc,hallyn/lxc,terceiro/lxc,hallyn/lxc,LynxChaus/lxc,aeris/lxc,caglar10ur/lxc,federicobriata/lxc,aeris/lxc,thmo/lxc,caglar10ur/lxc,terceiro/lxc,thmo/lxc,terceiro/lxc,LynxChaus/lxc,aeris/lxc,ss1h2a3tw/lxc,LynxChaus/lxc,caglar10ur/lxc,ss1h2a3tw/lxc,ss1h2a3tw/lxc,thmo/lxc