Summary:
On macOS, it appears that ssh has a ~1% chance of never being able to connect
too the server and just hang. This caused mactest to be completely unhealthy
for a couple of days and a similar hotfix was applied to mitigate the issue.
Since it proved to be working, let's now backport this hotfix in the actual
code.
Reviewed By: DurhamG
Differential Revision: D22953230
fbshipit-source-id: ead7662ea6d0a33efaa5c4044c9391b2835ee421