Fix downloader

This patch changes two things in install.pp
 - Switches the download tool from wget to curl to allow the use of
   the 'file://' scheme for the download URL (wget doesn't support file://)
 - Checks $::gitlab::puppet_manage_packages before declaring
   Package['curl']. This follows the behaviour of the other dependencies
   and Puppet best practices.
why not `nanliu/staging`?