Move LDAP config into PHP
Summary: This config section is weak (poorly documented) and inconsistent (keys with "_" instead of "-") but I'm going to keep punting on improving it until after T1536.
Test Plan: Loaded, examined LDAP config.
Reviewers: btrahan, codeblock
Reviewed By: codeblock
CC: aran
Maniphest Tasks: T2255
Differential Revision: https://secure.phabricator.com/D4322