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