REST test against clusters with dedicated master nodes

Today we run our REST tests against clusters with nodes that are both data nodes and master-eligible nodes. We therefore miss tests against the (recommended) configuration of dedicated master nodes and separate data nodes, and this includes things like tests of the rolling upgrade procedure which may be sensitive to the order in which nodes are upgraded.

Thanks to Armin for spotting this in https://github.com/elastic/elasticsearch/pull/34560#discussion_r225992140.