Scrub more variables when spawning java processes.

This adds scrubbing for `_JAVA_OPTIONS` and `JAVA_TOOL_OPTIONS`
which will quasi-silently add trumping jvm options to the command line
when present.  This leads to hermeticity leaks that can affect
correctness and are hard to debug.

Testing Done:
$ PANTS_DEV=1 ./pants goal test tests/python/pants_test/java:executor --test-pytest-options="-vs"
...
17:14:57 00:02   [test]
17:14:57 00:02     [pytest]
17:14:57 00:02       [run]
                     WARNING:root:Scrubbing CLASSPATH=/tmp/tmpnitp8W
                     WARNING:root:Scrubbing _JAVA_OPTIONS=-target 6
                     WARNING:root:Scrubbing JAVA_TOOL_OPTIONS=-Xmx1g
                     ============== test session starts ===============
                     platform linux3 -- Python 2.6.8 -- py-1.4.20 -- pytest-2.5.2 -- /home/jsirois/dev/3rdparty/pyenv/versions/2.6.8/bin/python
                     plugins: cov, timeout
                     collected 4 items

                     tests/python/pants_test/java/test_executor.py:47: SubprocessExecutorTest.test_not_scrubbed PASSED
                     tests/python/pants_test/java/test_executor.py:50: SubprocessExecutorTest.test_scrubbed_classpath PASSED
                     tests/python/pants_test/java/test_executor.py:54: SubprocessExecutorTest.test_scrubbed_java_options PASSED
                     tests/python/pants_test/java/test_executor.py:57: SubprocessExecutorTest.test_scrubbed_java_tool_options PASSED

                     ============ 4 passed in 0.15 seconds ============

17:15:02 00:07     [junit]
17:15:02 00:07     [specs]
               SUCCESS

Bugs closed: 260, 264

Reviewed at https://rbcommons.com/s/twitter/r/583/