Make default database namespace configurable

Summary: Allow the default namespace to be set in configuration, so you can juggle multiple copies of sandbox test data or whatever.

Test Plan: Changed default namespace, verified web UI and "storage" script respect it.

Reviewers: btrahan, vrana, jungejason

Reviewed By: vrana

CC: aran

Maniphest Tasks: T345

Differential Revision: https://secure.phabricator.com/D2341