Added --bg flag to polling commands

I screwed up. I thought it'd be easy to circle back and add this. What I didn't notice was that the main entry point into the command's functionality (poll()) didn't take the user input as a parameter. So the changes to polling.py are minimal to add the flag and use it. The rest of the changes are about a billion calls into poll that had to be changed to pass in the user input.

There will be a PR in pulp_rpm that corresponds to it. No changes needed to be made to pulp_puppet.