lxc-download: Tweak cache location

 - Also include the variant in the path
 - Fix invalid LXC_CACHE_BASE
 - Drop redundant code

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Implement lxc.rootfs.options

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>