Switch to metadata.json for Puppet forge releases

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

This patch :
 - convert the Modulefile to metadata.json.
 - bump major version to 5 (for juno).
 - switch to stackforge namespace.
 - relax pe version_requirement to 3.x.

Change-Id: I9c83b577887210486fa03ed358910e129069a57c