Pylint: fix different messages in pre-1.0/1.0

Pylint requires some messages disabled in 1.0+, but crashes if
these are given to pre 1.0. Use a dirty shell wrapper to fix :(.
fix some pylint warnings
NEWS: update
[download-scratch] Refactor into a function and add options

- Move functionality into a separate function that can be imported if needed
- Add optional --huburl and --baseurl for defining alternative koji instances
  for downloads (rhbz#1027616)
[download-scratch] Really use commandline argument for base url
[test] Initialize Null as new type class

This makes pylint happy
Add querying for installed external plugins into RegistryBase

This way we encourage consistent naming of external plugins and can simply
output messages to install additional plugins
java: move checks to external package

This way Java checks can be updated independently from fedora-review itself
Some remarks and sketches

This is just sketches, it probably doesn't pass the parser. Take it
for what it's worth. Until things are finalized, it would be nice
if the new java-plugin could sit in a separate dirwithin the tree so
it's possible to see that side as well.
Replace name property with external_plugin boolean

Deciding if plugin is external was the only use case for name anyway
Remove functions that will be unused

We will possibly add different get_build_id functions
Move is_plugin_installed into Checks class

Since we changed the approach a bit, registry is only concerned with itself.
However we need to ask inside basic checks if there is a registry for given group
with external_plugin set to True. That way we can create stub checks that will
output warnings about missing external plugins if needed.
[java] Use new Checks.external_plugin_installed