[polisher/fedora] Fetch user packages from pkdb api.

For starters, the previous `PACKAGE_LIST` was not working since pkgdb2 is using new uris, hence the method was broken. Then I thought since there is an [API](https://admin.fedoraproject.org/pkgdb/api/#user%27s_packages), why not use it?

So, `gems_owned_by(user)` now fetches a user's packages, both owned and co-maintained. If you wish to fetch only the ones a packager owns, I can simply remove the second query.

As for the tests, this method had [no tests](https://github.com/ManageIQ/polisher/blob/master/spec/fedora_spec.rb#L10) to begin with, but I'm not so familiar with rspec to test api queries. If anyone can point me how to test this, I'll happily implement it.
@axilleas I'd suggest you move the URL building, API open call, and JSON loading to a method if you want to test this.  This method would be responsible for returning a Hash.  You could then test a few different situations of the point of contact and co-maintained in the Hash.

The URL building, API open, and JSON loading method could then we tested independently, where only the URL building is probably necessary... (API open and JSON loading should be tested in the fedora API itself so there's no need to duplicate tests).