Fall back to LDAP search attribute if username attribute is not configured

Summary:
See discussion in D3340. Some configurations set only a search attribute because their records are indexed by username (this is probably not quite the correct LDAP terminology). Other configurations use one attribute to search and a different attribute to select usernames.

After D3340, installs which set only a search attribute broke. Instead, fall back to the search attribute if no username attribute is present.

Test Plan: Successfully logged in on my test slapd.

Reviewers: yunake, voldern, briancline

Reviewed By: voldern

CC: aran

Differential Revision: https://secure.phabricator.com/D3406