Add support for global LXC config files

This PR adds basic configuration of global LXC configuration parameters through the config files
  * `/etc/lxc/lxc.conf`
  * `/etc/lxc/default.conf`
  * `/etc/lxc/lxc-usernet`

Configuration example:

    {
      virtualisation = {
        lxc.enable = true;
        lxc.defaultConfig = ''
          lxc.network.type = veth
          lxc.network.link = virbr0
          lxc.network.flags = up
        '';
      };
    }