Allow custom exchange types

[CVTJNII]

It is possible to install other RabbitMQ exchange types via plugins. An example of this is the [https://github.com/rabbitmq/rabbitmq-consistent-hash-exchange](x-consistent-hash exchange) which I wish to use with Logstash to help with performance problems caused by high logging throughput. However, the output plugin validates the exchange type at rabbitmq.rb line 31 against only the three defaults, leading to the following error:

{:timestamp=>"2016-06-27T22:48:55.743000+0000", :message=>"Invalid setting for rabbitmq output plugin:\n\n  output {\n    rabbitmq {\n      # This setting must be a [\"fanout\", \"direct\", \"topic\"]\n      # Expected one of [\"fanout\", \"direct\", \"topic\"], got [\"x-consistent-hash\"]\n      exchange_type => \"x-consistent-hash\"\n      ...\n    }\n  }", :level=>:error}

It would be nice if this output was tolerant of other exchange types installed via Rabbit plugins.

• Version: logstash 2.2.4
• Operating System: logstash:2.2 Docker container
• Config File (if you have sensitive info, please remove it):

output {
  rabbitmq {
    exchange => "es"
    exchange_type => "x-consistent-hash"
    host => [Host]
    port => "5671"
    user => "es"
    password => [Password]
    vhost => "/es"
    ssl => true
  }
}

• Sample Data: N/A
• Steps to Reproduce: Attempt to start Logstash 2.2 with the above output configuration, failure is immediate

[CVTJNII]

Looks like the mixin supports this type in 4.0 which is in the latest container.