Install unattended-upgrades after Exec[refresh_apt]

Install unattended-upgrades after Exec[refresh_apt]

Before, including apt::unattended_upgrades on a host without
the unattended-upgrades package would fail on the first run,
because the module tries to install the package before apt is
finally configured.
This commit does:

- introduce the option $refresh_apt for apt::apt_conf
  (Defaults to true). Can be used to not trigger Exec['refresh_apt']
- install the unattended-upgrades package after a final
  Exec['refresh_apt']. To not run into a loop, it calls
  Apt_conf['50unattended-upgrades'] with the option
  refresh_apt => false, which is also not needed for the configuration