Allow bin/config to create config file

Summary:
See D20779, https://discourse.phabricator-community.org/t/3089. bin/config set complains about
missing config file as if it's un-writable.

Test Plan: run bin/config set with missing, writable, unwritable conf.json and parent dir.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D20788