Remove 'sub' parameter from Plugin.add_copy_spec*()

The 'sub' parameter to these functions allowed substituting part
of the path in the generated archive. E.g. transforming 'etc/'
into 'configs/'. This has never been used and seems to serve no
real purpose today. Simplify the parameter passing and copy_spec
lists by removing this support.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>