(#12809) $release should use $lsbdistcodename and fall back to manual input

(#12809) $release should use $lsbdistcodename and fall back to manual input

This commit changes $release to default to Facter's $lsbdistcodename
and fall back to a Parse Error if $release is not set and $lsbdistcodename
does not exist. Previously $release was hardcoded to karmic.

This commit also modifies apt::ppa to use $release and sets the
files to be ensured so that they are not purged when purge_sources_list_d
is set to true.