Expose configuration of data node types Master/HOT/WARM in the Data Node UI

## What?

Right now, it is possible to configure which nodes serve hot data, and warm data and which are masters, but only by setting `node.roles:` in the `datanode.conf` file. By default, all roles are assigned to all nodes, but the recommendation should be to have no more than three master nodes and decouple nodes which facilitate searching hot data and those which search warm data; the sensible way to do this would be to expose this configuration in the UI where the user can choose which nodes have which roles by labelling them.

## Why?

Requiring this option in the config file and not being apparent to the user could end up with sub-optimal cluster configurations resulting in poor performance.