Prepare for db-driven auth configuration by making proviers operate in dual modes

Summary:
Ref T1536. This sets us for the "Config -> Database" migration. Basically:

  - If stuff is defined in the database, respect the database stuff (no installs have anything defined yet since they can't reach the interfaces/code).
  - Otherwise, respect the config stuff (all installs currently do this).

Test Plan: Saw database stuff respected when database stuff was defined; saw config stuff respected otherwise.

Reviewers: chad, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6240