network.c: Add missing LXC_NET_NONE option + refactor

Add LXC_NET_NONE to known lxc_network_types, so parsing a config
file with lxc.network.type = none does not result in failure
(e.g. doc/examples/lxc-no-netns.conf). Options have also been
reordered to match the enum in conf.h.

Signed-off-by: Bogdan Purcareata <bogdan.purcareata@freescale.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>