Rakefile: Use Puppetfile from openstack-puppet-modules

puppet-openstack-cloud is moving to rely on
openstack-puppet-modules[1] to manage puppet modules dependency.
Hence, there is no need to have a Puppetfile locally, tests will be
fetching the Puppetfile directly from openstack-puppet-modules.

[1] https://github.com/redhat-openstack/openstack-puppet-modules

Change-Id: Icb3cd590b29b69e0b9b7d9b7b09309353b1305a7
Merge "Rakefile: Use Puppetfile from openstack-puppet-modules"