test: use the correct number of bwc nodes for old cluster

qa/rolling-upgrade/build.gradle

@@ -26,7 +26,7 @@ task oldClusterTest(type: RestIntegTestTask) {
   cluster {
     distribution = 'zip'
     bwcVersion = '6.0.0-alpha1-SNAPSHOT' // TODO: either randomize, or make this settable with sysprop
-    numBwcNodes = 1
+    numBwcNodes = 2
     numNodes = 2
     clusterName = 'rolling-upgrade'
   }