Add support for changing the order of processors

[stevenpost]

Hi,

Changing the order of processors allows to hook up pipelines to streams, instead of going through pipelines first.
I did some digging in the API for Graylog.

API endpoint: /api/system/cluster_config/org.graylog2.messageprocessors.MessageProcessorsConfig
To get the current config: GET
To update the config: PUT

The response data (for both methods) is the same as the data to send for an update:
Example

{
  "processor_order": [
    "org.graylog.aws.processors.instancelookup.AWSInstanceNameLookupProcessor",
    "org.graylog.plugins.map.geoip.processor.GeoIpProcessor",
    "org.graylog2.messageprocessors.MessageFilterChainProcessor",
    "org.graylog.plugins.pipelineprocessor.processors.PipelineInterpreter"
  ],
  "disabled_processors": []
}