Node naming support for the cluster framework

[ckreibich]

The cluster framework's Node record to captures information about a node in the cluster. The record includes ip: addr to capture the node's IP address. In modern environments (think docker-compose) the IP address is ephemeral, with hosts generally referred to by name. Broker can already deal transparently with string values that convey names or IP addresses.

We should transition the Node record to string identifiers, like host: string. This seems pretty straightforward — introduce that field alongside the current one, deprecate the former, discard the former in 6.1.

[perkinjo]

I ran into the current limitation of needing ip addresses instead of hostnames recently when trying to orchestrate some tests with docker compose. It'd be easier to be able to use hostnames for a set of tools where the ip addresses are not known ahead of time but we do know the hostnames.