Avoid variable lookup on apt::unattended_upgrades

Avoid variable lookup on apt::unattended_upgrades

The current lookup for '::apt::custom_preferences' might lead to a lookup error
like

  (Scope(Class[Apt::Unattended_upgrades])) Could not look up qualified variable
  '::apt::custom_preferences'; class ::apt has not been evaluated at
  /etc/puppet/modules/apt/manifests/unattended_upgrades.pp:17

Since the lookup apparently happens during compilation time, this commits tries
to fix this issue by using a define() function call instead of the lookup.