Additional LDAP Authentication Methods (#1287)

This patch allows anonymous and user based authentication to LDAP
servers. This change is based on a patch against bn-ldap-authentication:

-  https://github.com/blindsidenetworks/bn-ldap-authentication/pull/2

The patch introduces a new environment variable `LDAP_AUTH` which
controls the authentication method used against the LDAP server:

- `anonymous` enables an anonymous bind to the LDAP with no password
  being used.

- `user` uses the user's own credentials to search for his data,
  enabling authenticated login to LDAP without the need for a user with
  global read privileges.

The default still remains at using a bind user, allowing for a seamless
upgrade path from the previous version.

This fixes #1082

Co-authored-by: Jesus Federico <jesus@123it.ca>

app/controllers/sessions_controller.rb

@@ -131,6 +131,7 @@ class SessionsController < ApplicationController
     ldap_config[:port] = ENV['LDAP_PORT'].to_i != 0 ? ENV['LDAP_PORT'].to_i : 389
     ldap_config[:bind_dn] = ENV['LDAP_BIND_DN']
     ldap_config[:password] = ENV['LDAP_PASSWORD']
+    ldap_config[:auth_method] = ENV['LDAP_AUTH']
     ldap_config[:encryption] = if ENV['LDAP_METHOD'] == 'ssl'
                                     'simple_tls'
                                 elsif ENV['LDAP_METHOD'] == 'tls'