Add default mirrors and sources for Raspbian.

Because ansible is (as of today) not able to detect Raspbian,
you will still have to set::
  apt_default_sources_lookup: 'raspbian'
  apt_default_mirrors_lookup: 'raspbian'
in your host's `host_vars/xxx.yml`.
Merge pull request #11 from htgoebel/master

Add default mirrors and sources for Raspbian, see issue #9.