Moves all remaining (rolling-upgrade and mixed-version) REST tests to use Zen2. To avoid adding
extra configuration, it relies on Zen2 being set as the default discovery type. This required a few
smaller changes in other tests. I've removed AzureMinimumMasterNodesTests which tests Zen1
functionality and dates from a time where host providers were not configurable and each cloud
plugin had its own discovery.type, subclassing the ZenDiscovery class. I've also adapted a few tests
which were unnecessarily adding addTestZenDiscovery = false for the same legacy reasons. Finally,
this also moves the unconfigured-node-name REST test to Zen2, testing the auto-bootstrapping
functionality in development mode when no discovery configuration is provided.
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