Add an option to disable LDAP encryption

We had one use case where the LDAP server didn't support encryption,
so support that too.

CHANGELOG.md

+# v0.3.3
+
+* config: ldap.encryption gained an option "plain" for disabling encryption

src/DependencyInjection/Configuration.php

@@ -25,8 +25,8 @@ class Configuration implements ConfigurationInterface
             ->scalarNode('username')->end()
             ->scalarNode('password')->end()
             ->enumNode('encryption')
-                ->info('simple_tls uses port 636 and is sometimes referred to as "SSL", start_tls uses port 389 and is sometimes referred to as "TLS"')
-                ->values(['start_tls', 'simple_tls'])
+                ->info('simple_tls uses port 636 and is sometimes referred to as "SSL", start_tls uses port 389 and is sometimes referred to as "TLS", plain means none')
+                ->values(['start_tls', 'simple_tls', 'plain'])
                 ->defaultValue('start_tls')
             ->end()
             ->end();

src/Service/LDAPApi.php

@@ -96,10 +96,10 @@ class LDAPApi implements LoggerAwareInterface, ServiceSubscriberInterface
         ];
 
         $encryption = $config['ldap']['encryption'];
-        assert(in_array($encryption, ['start_tls', 'simple_tls'], true));
+        assert(in_array($encryption, ['start_tls', 'simple_tls', 'plain'], true));
         $this->providerConfig['use_tls'] = ($encryption === 'start_tls');
         $this->providerConfig['use_ssl'] = ($encryption === 'simple_tls');
-        $this->providerConfig['port'] = ($encryption === 'start_tls') ? 389 : 636;
+        $this->providerConfig['port'] = ($encryption === 'start_tls' || $encryption === 'plain') ? 389 : 636;
     }
 
     public function checkConnection()