Add lxc.default_config to LXC's own config

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Re-organize API for global lxc.conf config

Instead of having one function for each possible key in lxc.conf which
doesn't really scale and requires an API update for every new key,
switch to a generic lxc_get_global_config_item() function which takes a
key name as argument.

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