BWC testing: remove hardcoded OpenSearch version

In order to remove hardcoded version of actual OpenSearch from BWC versions array we
need to introduce new 'empty' property and update it before testClusters are defined.

Closes: #324

Signed-off-by: Lukáš Vlček <lukas.vlcek@aiven.io>

build.gradle

@@ -174,11 +174,15 @@ task copyZIPBundle {
     }
 }
 
+// A hack. For some reason we can not reference 'ext' variables from testClusters, hence we define actual opensearch
+// version the following way which seems to work fine for testClusters. See issue #324
+project.opensearch_version = ext.versions.opensearch
+
 // Clusters for BWC tests
 2.times { i ->
     testClusters {
         "${baseName}$i" {
-            versions = [project.BWCversion, "2.17.1"]
+            versions = [project.BWCversion, project.opensearch_version]
             numberOfNodes = 3
             plugin(provider(new Callable<RegularFile>() {
                 @Override

gradle.properties

@@ -3,6 +3,9 @@
 # An actual version of plugin
 version = 2.17.1.0
 
+# Leave this property empty, it is assigned during the gradle build execution (yes, it is a hack! see issue #324)
+opensearch_version =
+
 # A version of OpenSearch cluster to run BWC tests against
 BWCversion = 2.17.0