Changes to support displaying copied units in the CLI

Corresponds to: https://github.com/pulp/pulp/pull/415

There was already a hook in PollingCommand to handle the task end result. This was implemented to use a separate module for rendering the units. That rendering is kept separate because my next round of changes will be to the unassociate units flow to return the units as well, so that rendering is going to be needed for that command as well.

If you're wondering what the deleted assertions are, they are a carry over from before all of the logic was moved into PollingCommand. Those tests verify the correct output, so I'm not concerned with replicating that (and having to maintain those assertions) here as well.