<LDAP: If available use DN from previously retrieved user>

Summary:
When searching for a user before logging in use the DN from the retrived user.
This allows you to use a less fine grained DN when searching for a user. For example dc=domain,dc=domain instead of ou=unit,dc=domain,dc=com.

Test Plan: Tested on local install with ldap.search-first disabled and enabled.

Reviewers: epriestley, yunake

Reviewed By: epriestley

CC: auduny, briancline, aran, Korvin, vsuba

Differential Revision: https://secure.phabricator.com/D3549