Adds apt::{conf,backports} and variable style.

Adds apt::{conf,backports} and variable style.

  With the addition of this patch two new defines will be added; one to
  manage APT configuration files (apt::conf) and one that abstracts out the
  requirements needed to turn on backport repositories (apt::backports).

  In addition, the patch takes the opportunity to clean up variable
  definitions so they follow a consistent pattern of setting local
  variables to the fully qualified value stored in the apt::params
  class.  Previously all variable used within a class directly addressed
  the apt::params namespace when ever the variable was used.  In the
  pattern they now adhere to we can more easily switch the namespace
  data lives in or externalize it even more using hiera.