make 'empty network' the default

Currently if no lxc.network.type section is in the container
configuration, the container ends up sharing the host's network.
This is a dangerous default.

Instead, add 'lxc.network.type = none' as a valid type, and make
en empty network the default.

If none as well as another network type are specified, then the
none type will be ignored.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>