Build: Change tar extension back to tar.gz instead of tgz

Gradle defaults to tgz extension when tar is compressed. This changes
the tar distribution back to tar.gz. Note that this also means the maven
packaging type is now tar.gz.

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

@@ -235,7 +235,7 @@ class ClusterFormationTasks {
 
     static void configureDistributionDependency(Project project, String distro) {
         String elasticsearchVersion = ElasticsearchProperties.version
-        String packaging = distro == 'tar' ? 'tgz' : distro
+        String packaging = distro == 'tar' ? 'tar.gz' : distro
         project.configurations {
             elasticsearchDistro
         }

distribution/tar/build.gradle

@@ -19,6 +19,7 @@
 
 task buildTar(type: Tar, dependsOn: dependencyFiles) {
   baseName = 'elasticsearch'
+  extension = 'tar.gz'
   with archivesFiles
   compression = Compression.GZIP
 }