Added plugin utility for going between importer config and downloader

config

I added pulp.plugins.util.downloader (to be renamed when we get a name) with the logic to go from Pulp standard importer config to downloader config. That way, it's accessible to all importers and everyone isn't doing that translation.

The other change was to PluginCallConfiguration to get a single dict view on the configuration values so you have something to pass into this call. There weren't, or I couldn't find, unit tests for it, so I added a test case for that new method as well.