Make a copy of the lib/logstash/outputs/elasticsearch/elasticsearch-template.json from your Logstash distribution (possibly installed as /opt/logstash/lib/logstash/outputs/elasticsearch/elasticsearch-template.json), modify it by replacing

"dynamic_templates" : [ {
  "string_fields" : {
    "match" : "*",
    "match_mapping_type" : "string",
    "mapping" : {
      "type" : "string", "index" : "analyzed", "omit_norms" : true,
      "fields" : {
        "raw" : {"type": "string", "index" : "not_analyzed", "ignore_above" : 256}
      }
    }
  }
} ],

with

"dynamic_templates" : [ {
  "string_fields" : {
    "match" : "*",
    "match_mapping_type" : "string",
    "mapping" : {
      "type" : "string", "index" : "not_analyzed", "omit_norms" : true
    }
  }
} ],

and point template for you output plugin to your modified file:

output {
  elasticsearch {
    ...
    template => "/path/to/my-elasticsearch-template.json"
  }
}

You can still override this default for particular fields.