Many enchancements to apt cache host servers

- the way that apt cache hosts are defined has changed. Now apt cache is not
  enabled by default, to enable it, set apt_cache_host variable with FQDN name
  of the cache host, in group_vars/ (per group) or host_vars/ (per host);

- 'webfs' package has been replaced by 'nginx' role, which resolves problems
  with conflicting packages in case nginx is later installed on the same host;

- nginx configuration is defined inside 'apt' role. Nginx default server
  configuration is disabled by the role on hosts which serve as apt caches;

- nginx on apt cache hosts is used to serve Debian Preseed configuration. You
  can use it within Debian-Installer by adding 'url=apt_cache_host' on the
  boot command line. Custom Debian repositories are planned in the future;

- apt cache hosts install and setup TFTP server with Debian Netboot
  configurtion which can be used (after pointing to a cache server in DHCP) to
  boot Debian-Installer using PXE. In the future Debian-Installer will be
  modified to provide ansible-aiua-related options in the installer menu;

- access to apt-cacher-ng and TFTP server is restricted by default to local
  LAN hosts, can be overriden, added to, or disabled using variables;