[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 @movitto I think it makes sense to have one place that converts a RPM package name into rubygems gem name?  We do this twice here.  I vaguely recall there are other places that do this "starts with rubygem-" and/or "remove leading rubygems-" string querying/manipulation. 

Maybe a RpmPackageName class with .rubygem? and .rubygem_name methods that do this detection/rubygem- removal....

Just a thought.