Add add_cmd_outputs() and convert plugins to use it

Add a new method to Plugin to add a list of commands to be run,
analogous to add_copy_specs() vs add_copy_spec().

Also convert a few plugins not using add_copy_specs() for long
file lists (or using it inconsistently).

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