Fix Kernel#open warning downloading repo tarball

```
lib/manageiq/cross_repo/repository.rb:151: warning: calling URI.open via Kernel#open is deprecated, call URI.open directly or use URI#open
```

Author: agrare

lib/manageiq/cross_repo/repository.rb

@@ -148,7 +148,7 @@ def tarball_url
     end
 
     def open_tarball_url(url)
-      archive = open(tarball_url, "rb")
+      archive = URI.open(tarball_url, "rb")
 
       if archive.kind_of?(StringIO)
         archive = Tempfile.new('cross_repo').tap do |f|