assemble-public-tools inflates ppa downloads counts

[Curtis Hovey]

assemble-public-tools downloads debs from many sources:
   * ubuntu archive
   * ports.ubuntu.com
   * stable and devel public ppas
   * stable and devel building ppas

jerff runs the script many times, each to locate new tools. Though it isn't finding new tools, the downloads are being counted. This inflates the downloads from the ppas where we count user adoption by client downloads.

While the script needs to downloads from all these sources, there is no case were a single run needs to check every single source.
   * During a release, the release manager only checks the private ppa where the tools were built
   * Jerf only needs to check Ubuntu and locate additional tools created after the release.
   * Private clouds many want to check the stable and devel public ppas, this isn't proven

We could add switches to select the source to search: --ubuntu, --juju-packaging and maybe --juju
* jerff would use the --ubuntu
* the release manager would use --juju-packaging

The more I think about this, the less I see a need to the public ppas, They were superseded by the private ppas. No user needs to make their own tools from the packages in the public ppas because that would create a duplicate with a different checksum caused by a change in timestamps in the tgz.