Remove debug_args from pants.ini.  Fixes issue 650 by removing string interpolation of debug_port

Testing Done:
Added a unit test for new jvm_debug_config.py

Ran the following tests with `debug_port` unset and set in pants.ini to make sure debug port was actually overridden

./pants goal test ./examples/tests/java/com/pants/examples/hello/greet --test-junit-debug
./pants goal test ./examples/tests/java/com/pants/examples/hello/greet --test-junit-debug --test-junit-coverage --test-junit-coverage-processor=emma
./pants goal test ./examples/tests/java/com/pants/examples/hello/greet --test-junit-debug --test-junit-coverage --test-junit-coverage-processor=cobertura
./pants goal run testprojects/src/java/com/pants/testproject/unicode/main --run-jvm-run-debug
./pants goal idea ./src:: # checked the project.i* file for the debug port

I could not find examples or testprojects for goals benchmark and spec to test them out.

Bugs closed: 650, 652

Reviewed at https://rbcommons.com/s/twitter/r/1137/
Sync of https://github.com/pantsbuild/pants/commit/ad3cad69400847621f9a8fef8fd9f680c7eac3c5