archlinux template: fix lxc.root for btrfs backend

when using btrfs backend lxc-create first creates rootfs in /usr/lib/lxc/rootfs
directory before moving it to /var/lib/lxc or other directory supplied by the
command line. Archlinux template relied in $rootfs_path which made containers
created with btrfs backend have lxc.rootfs set to /usr/lib/lxc/rootfs. By using
$path instead of $rootfs_path we make sure that lxc.rootfs is always correct.