Added parser and modifier for /etc/hosts

- Removed dirty hack job to update /etc/hosts and /etc/hostname via a
  mount hook script by etchosts module
- Enabled update of the /etc/hosts file on the lxc host system
- Reformatted argparse arguments
- Restructured Locker features in groups in README
- Got rid of container._link_container()
- Only query containers for IPv4 addresses