Migrate some reasonable default values from pants.ini into 'defaults' in the pants source.  This covers scala-compile, jvm-compile, protobuf-gen, and a few others.

Testing Done:
CI is baking at: https://travis-ci.org/pantsbuild/pants/builds/32014416

Bugs closed: 455, 456, 458

Reviewed at https://rbcommons.com/s/twitter/r/852/