Use @tar_host for shipping tarballs, not @yum_host

This is in another effort to fix dumb mistakes. Early on, we just assumed
tarballs would go to @yum_host (why? probably just laziness) but this is not
ideal and does not make any sense when looking at the code. Now there's a
@tar_host variable, but for backwards compatibility, we'll default back to
@yum_host if @tar_host isn't set.

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