Address additional review of tar.rb/tar.rake

* In Pkg::Tar
 - There was a mix of where the tarball class gets its data loaded. Some got
   loaded in the class itself from Pkg::Config, and some got loaded from the
   tar.rake rake task. To make it consistent, the data should get loaded within
   one or the other, and for our purposes loading the data in tar class is
   probably best. This commit moves the loading and type checking into the tar
   class for excludes and templates.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>
Make redhat spec extraction more efficient

Rather than unpacking the whole tarball just to check if the redhat spec
exists, we could instead just grep the tarball file list for it, and then
extract just that. This is way more efficient, and this commit makes it so.

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