Rearrange network infrastructure plays and roles

- old 'debops.nat' role has been obsoleted and will be removed from
  GitHub and Ansible Galaxy. It has been replaced by 'debops.subnetwork'
  role;

- new 'debops.radvd' role has been added and manages 'radvd', IPv6
  Router Advertisement daemon;

- KVM and LXC plays in 'virtualization.yml' playbook have been modified
  and they no longer automatically configure internal network and dnsmasq,
  just their respective services (KVM and LXC);

- network infrastructure plays (DHCP/dnsmasq and internal network) have
  been moved to a separate playbook, 'networking.yml';
Update CHANGELOG.rst
Merge pull request #46 from drybjed/networking-playbook

New 'networking.yml' playbook and role changes