Add ability to change cluster configuration settings in terraform provider

[isaacegglestone]

API Endpoints

/api/system/cluster_config

My specific use case is for: /api/system/cluster_config/org.graylog2.messageprocessors.MessageProcessorsConfig

This would be a normal use case because pipelines are set to process before message filter chain and many people need it to come after the filter chain.

Ideally we could manage anything that shows up in the GUI in: /system/configurations

And would be able to set other things like relative time ranges as well.

See the reference on this page about message processing order: https://docs.graylog.org/en/3.1/pages/pipelines/stream_connections.html