Support Redis configuration (hashicorp#1706)

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/cc @edevil 

Fixes hashicorp#1547

Author: modular-magician

docs/r/redis_instance.html.markdown

@@ -98,6 +98,9 @@ The following arguments are supported:
 * `labels` -
   (Optional)
   Resource labels to represent user provided metadata.
+* `redis_configs` -
+  (Optional)
+  Redis configuration parameters, according to http://redis.io/topics/config.
 * `location_id` -
   (Optional)
   The zone where the instance will be provisioned. If not provided,