Switch to metadata.json for Puppet forge releases

Since puppet 3.x, Puppet Labs Forge accept only the metadata.json file
for module description (Modulefile is now deprecated).

This patch :
 - convert the Modulefile to metadata.json.
 - switch to stackforge namespace.
 - relax pe version_requirement to 3.x.
 - added a common gitignore (ignore pkg)

Change-Id: Idc82bed42bc2955bd5f46dd2bd80fff13542d97b