Migrate gemspec info in project_data.yaml to the .gemspec file

We currently use packaging's "package:gem" task to build the Puppet gem which adds a lot more complexity than we need (i.e. it builds a gemspec from data in project_data.yaml and then builds the corresponding default gem from it + any platform-specific gems [that might have platform-specific dependences]).

We should have the info. in project_data.yaml in the .gemspec file so that a simple gem build should do the trick.