Added display of removed units, refactoring copy units a bit in the

process

The actual change is two lines, simply the implementation of the succeeded hook and pass it to the existing display_units call. I pulled out a base class for the remove commands to define that and tweaked the tests accordingly. There were a few tweaks to the unit display method itself because as forward thinking as I thought I was being when I wrote it for copy, I still had some copy-specific verbiage in there.

Corresponds to: https://github.com/pulp/pulp/pull/416