965751 - migrate pulp to use the threaded downloader instead of curl.

https://bugzilla.redhat.com/show_bug.cgi?id=965751

Few things:
1. changed import in the importer to use the threaded downloaded.
2. needed to split the ssl key & cert because the curl downloader handles the key and cert in the cert property and the threaded downloader does not.
3. needed to update the unit tests to use the local file downloader because the test use file:// URLs which are supported by the curl downloader but not the threaded downloader.
4. moved creating the destination download directories when building the requests instead of in the download_started() callback.  The local file downloader does not call this callback before attempting to open the file so unit tests were failing.  Even though the local downloader will be fixed at some point, where I moved it is just as good.
5. simplified a few things (housekeeping) in the unit tests.