R1:318e8ebdac95

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