(#FORGE-38) Add Rakefile to skeleton for generated modules

This patch adds a simple Rakefile that provides rake tasks `lint` for
puppet-lint, `validate` for validation of manifests, templates, and ruby
files, and the tasks provided by the puppetlabs_spec_helper.
Merge pull request #2254 from ghoneycutt/FORGE38_module_generate_skeleton_rakefile

(PUP-2045) (FORGE-38) Add Rakefile to skeleton for generated modules