allow watchman to run with no configuration file
allow watchman to run with no configuration file

Summary: to address https://github.com/facebook/watchman/issues/905, the old behavior allowed there to not be a `/etc/watchman.json`, we should also allow there not to be a `/etc/watchman.json.default`

Reviewed By: chadaustin

Differential Revision: D27914269

fbshipit-source-id: 5771310c30e3920e4edda64e6513e53fe17bd6b1

nodakai/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman