GH-9: Add rackspace mirror

Add secondary mirror that itself redirects to various locations:
https://www.archlinux.org/mirrors/rackspace.com/

rootfs/etc/pacman.d/mirrorlist

 Server = https://mirror.leaseweb.net/archlinux/$repo/os/$arch
+Server = https://mirror.rackspace.com/archlinux/$repo/os/$arch