Platform agent robustness2

More negative testing:
- test_with_leaf_subplatform_directly_stopped
- test_with_intermediate_subplatform_directly_stopped

The leaked processes in the latter test:
```
bin/nosetests -sv --nologcapture --with-processleak --with-pycc ion/agents/platform/test/test_platform_agent_robustness.py:TestPlatformRobustness.test_with_intermediate_subplatform_directly_stopped
...
Process leak report
Test                                                                       Leaked Processes
========================================================================== ==================================================================
TestPlatformRobustness.test_with_intermediate_subplatform_directly_stopped
                                                                           ('PlatformAgent_LJ01B1e97fe2656984d41ac58c5ce75f31208', 'RUNNING')
                                                                           ('PlatformAgent_MJ01Bdf3ffb7e10ed4e649bdc437a14b5fc9f', 'RUNNING')
```

is a known issue.  See comment in the code.

@edwardhunter please review/merge

@bobfrat FYI