Set minimum_master_nodes to all nodes for REST tests (#27344)

PR #26911 set minimum_master_nodes from number_of_nodes to (number_of_nodes / 2) + 1 in our REST tests. This has led to test failures (see #27233) as the REST tests only configure the first node in its unicast.hosts pinging list (see explanation here: #27233 (comment)). Until we have a proper fix for this, I'm reverting the change in #26911.

buildSrc/src/main/groovy/org/elasticsearch/gradle/test/ClusterConfiguration.groovy

@@ -64,10 +64,10 @@ class ClusterConfiguration {
 
     /**
      * Configuration of the setting <tt>discovery.zen.minimum_master_nodes</tt> on the nodes.
-     * In case of more than one node, this defaults to (number of nodes / 2) + 1
+     * In case of more than one node, this defaults to the number of nodes
      */
     @Input
-    Closure<Integer> minimumMasterNodes = { getNumNodes() > 1 ? getNumNodes().intdiv(2) + 1 : -1 }
+    Closure<Integer> minimumMasterNodes = { getNumNodes() > 1 ? getNumNodes() : -1 }
 
     @Input
     String jvmArgs = "-Xms" + System.getProperty('tests.heap.size', '512m') +