Address review comments in tar.rb

* In Pkg::Tar
 - Rename variable (t) to (template_file) for easy reading
 - Rename variable (rel_path_to_erb) to (rel_path_to_template) for easy reading
 - use 'cd' instead of 'Dir.chdir' since we've included fileutils. It's just cleaner.
 - Wrap all paths in our shell execution of `tar` in single quotes for safety

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>