Today we pass `discovery.zen.minimum_master_nodes` to nodes started up in
tests, but for 7.x nodes this setting is not required as it has no effect.
This commit removes this setting so that nodes are started with more realistic
configurations, and deprecates it.
This commit overhauls the documentation of discovery and cluster coordination,
removing mention of the Zen Discovery module and replacing it with docs for the
new cluster coordination mechanism introduced in 7.0.
Relates #32006