Rewrite support for APT repositories

- new version of APT repository management supports multiple APT
  mirrors, different distributions and user-specified APT repositories;

- new set of variables has been added to 'debops.apt' defaults:

  'apt_mirrors_*' controls APT mirror URIs used in default or custom
                  repositories

  'apt_sources_*' controls APT repositories (default and user-specified)

- default APT repository configuration has been moved from a separate
  file in '/etc/apt/sources.list.d/*' directory directly into
  '/etc/apt/sources.list' which is automatically generated (old version of
  the file is backed up using 'dpkg-divert');

- two sets of APT repositories are included, one for Debian GNU/Linux,
  and one for Ubuntu Linux distributions. If Ansible cannot recognize
  a Linux distribution (checked using 'ansible_distribution' variable),
  it will use the default /etc/apt/sources.list file already configured
  on the host;
Change preseed config to use separate variables

This will make sure that Debian Preseed configuration will work using
old-style repository definition (just a hostname, not full URI).
Merge pull request #6 from drybjed/apt-updates

Apt updates