Migrate ensime_gen.py, reporting_server.py to new options format.

Testing Done:
manually invoking tasks w/ command line flags.
ci is running

Bugs closed: 746

Reviewed at https://rbcommons.com/s/twitter/r/1251/
Sync of https://github.com/pantsbuild/pants/commit/19a466633ea3e445d6bed880171b6e3a81d54c5d