roles/nextcloud: Use Redis for caching

The Nextcloud community [recommends][0] using Redis as a cache provider,
to improve response times and file locking reliability.

roles/nextcloud/meta/main.yml

@@ -1,4 +1,7 @@
 dependencies:
+- role: redis
+  tags:
+  - redis
 - role: cert
   vars:
     cert_src: lego/_.pyrocufflink.net.crt

roles/nextcloud/templates/config.php.j2

@@ -49,4 +49,13 @@ $CONFIG = array (
   'maintenance' => {{ nextcloud_maintenance|bool|string|lower }},
   'theme' => '',
   'loglevel' => 2,
+  'filelocking.enabled' => true,
+  'memcache.distributed' => '\OC\Memcache\Redis',
+  'memcache.locking' => '\OC\Memcache\Redis',
+  'redis' =>
+  array  (
+    'host' => 'localhost',
+    'port' => 6379,
+    'timeout' => 0.0,
+  ),
 );