Rewritten apt-cache-ng support for 'apt' role

- apt cache is not enabled by default, to enable it, you have to set
  'apt_cache_host' variable to value of ansible_fqdn of a cache server;

- apt-cacher-ng will be installed and configured on specified servers
  before further APT configuration. Also access through firewall will be
  enabled/disabled at correct time;

- other hosts will be automatically configured according to their group or
  host variables to use specified cache server. For example you can define
  one cache server in 'group_vars/all' and specific cache servers (or disabled)
  in other groups, for example for datacenters in different locations;

- apt-cacher-ng configuration file is now much shorter and cleaner (comments
  not pasted in finished template);