Fix autoscaling internal cluster release tests

This commit addresses an issue with the autoscaling feature flag not
being registered in release builds of the internal cluster tests. This
commit addresses this by enabling the system property that is needed,
but only in release builds.

x-pack/plugin/autoscaling/build.gradle

@@ -1,3 +1,5 @@
+import org.elasticsearch.gradle.info.BuildParams
+
 evaluationDependsOn(xpackModule('core'))
 
 apply plugin: 'elasticsearch.esplugin'
@@ -20,6 +22,9 @@ task internalClusterTest(type: Test) {
 
   include '**/*IT.class'
   systemProperty 'es.set.netty.runtime.available.processors', 'false'
+  if (BuildParams.isSnapshotBuild() == false) {
+    systemProperty 'es.autoscaling_feature_flag_registered', 'true'
+  }
 }
 
 check.dependsOn internalClusterTest