Added class lxc::networking::containers.

* This class will be used to configure default networking settings for
  the containers. Using lxc_interface type still allows to configure
  containers' specific settings.
Switched vagrant image with puppet included
Converted subclasses to private.

* All lxc::networking::* classes are now private.
* Logic is now in init.pp.
* The module can control NAT'ed network and lxc.network.* defaults.
* Added unit tests for all the changes.
* Added templates lxc-net and dnsmasq.conf.erb.
* Added default values for networking parameters.