Changed unit association API to return the units copied. Tweaks to the

PollingCommand based on live usage.

The first batch of changes were to signatures in PollingCommand. Most of that is unrelated to this change but was noticed when I started tweaking the copy output.

The second major area of change was in the Importer API to have the plugin return the list of copied units. The association manager then returns a serializable version of that list to be stored in the task and thus made accessible to client-side commands.

There will be PRs in both pulp_rpm and pulp_puppet that correspond to this change.