parallelize_bmuf_distributed_test intermittently hangs

Example: https://ci.pytorch.org/jenkins/job/caffe2-builds/job/py2-gcc5-ubuntu16.04-test/6806/console

```
23:45:38 lib/python2.7/dist-packages/caffe2/python/parallelize_bmuf_distributed_test.py::DistributedTest::test_bmuf_distributed Build timed out (after 45 minutes). Marking the build as failed.
00:27:43 Build was aborted
```

Note the timestamps.

@enosair, could you please look at it?