test_multiprocessing fails on OS X

The 'ps' command on OS X does not accept a "-ppid" argument and _get_children() returns an empty string.

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ERROR: glance.tests.functional.v1.test_multiprocessing.TestMultiprocessing.test_interrupt_avoids_respawn_storm
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_StringException: Traceback (most recent call last):
  File "/Users/bde/glance/glance/tests/functional/v1/test_multiprocessing.py", line 63, in test_interrupt_avoids_respawn_storm
    execute(cmd, raise_error=True)
  File "/Users/bde/glance/glance/tests/utils.py", line 271, in execute
    raise RuntimeError(msg)
RuntimeError: Command kill -INT did not succeed. Returned an exit code of 1.

STDOUT:

STDERR: kill: usage: kill [-s sigspec | -n signum | -sigspec] pid | jobspec ... or kill -l [sigspec]