Migrate checkstyle's use of self.config to self.get_options() for fetching from pants.ini.

The options names should be backwards compatible.  However, the task doesn't seem to be wired in to any
goal at the moment, so I can't add a section name to the migration script.

Testing Done:
CI passed.

Bugs closed: 828

Reviewed at https://rbcommons.com/s/twitter/r/1399/
Sync of https://github.com/pantsbuild/pants/commit/af22a8ba607c71b9daa76a93c36fa41e372842dc