Load LDAP provider correctly in `bin/auth` LDAP debug script

Summary: Ref T1536. After DB-driven auth config, we need to load this differently.

Test Plan: Ran `bin/auth ldap`.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6287