Allow Datanode Roles

Currently all Datanodes have all roles, `data`, `control` and `search`. Larger clusters will need a way to give Datanode's specific roles to delegate the cluster responsibilities.

**Describe the solution you'd like**
There should be a clear way to dedicate a number of node-groups. Each group able to take responsibility for a mix of roles or just a single role.

**Additional context**
This request borders on the duties of a Kubernetes operator, but explore what knobs we have to turn to enable this.

Start by reviewing the graylog-helm chart's values and templates to identify the available node-group and Datanode configuration knobs. Compare those options with the Kubernetes operator responsibilities described in the issue. Done means the chart provides a clear, documented way to assign data, control, and search roles to dedicated or mixed node-groups.

Written by the indexing model from the issue text.