(#15464) Make contributing easy via bundle Gemfile

Without this patch the process of figuring out how to quickly set up a
development and testing environment for the Puppet _application_ (not a
gem library) is unnecessarily complicated.

This patch addresses the problem by providing a Bundler compatible
Gemfile that specifies all of the Gem dependencies for the Puppet 2.7
application.

Puppet contributors can now easily get a working development and testing
environment using this sequence of commands:

$ git clone --branch 2.7.x git://github.com/puppetlabs/puppet.git
$ cd puppet/
$ bundle install # Install all required dependencies
$ rspec

The .noexec.yaml file excludes the rake command so that the Gemfile
doesn't raise an exception if the rubygems-bundler Gem is installed
and automatically running bundle exec for us.

The Gemfile.lock contains the exact dependency versions. This file is
included in the version control system because we're treating Puppet as
an application rather than a library.