Enable support for domain specific drivers, managed via
configuration files (instead of directly using the API and
database).

Using multiple domains means that calls to users.list must
be scoped to a specific domain; ensure that v3 calls to this
method are appropriately scoped.

Change-Id: I7ed84b7210597ab1633eba343a0c68741a5a8578
Partial-Bug: 1645803