Accept a list of node_basenames for the DNSPoll strategy

I would like to give different basenames to the various machines in my (cross-region) cluster, to make it more obvious which node(s) are running in which region.

This will however not work with the DNSPoll strategy because it expects a single node_basename to be configured.

I think it could easily be adapted to support multiple node_basenames, by allowing the `node_basename:` field to accept a list of binaries (as well as as single binary as it does now).