allow site-wide customization of zfsroot and lvm vg

/etc/lxc/lxc.conf can contain

	zfsroot = custom1
	lvm_vg = vg0

(Otherwise the defaults are 'lxc' for lvm_vg, and 'lxc' for zfsroot)

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>