build: fix the artifactId in the pom for x-pack-transport

This changes the pom generation to use the value of archivesBaseName when generating
the pom file.

Closes elastic/elasticsearch#3668

Original commit: elastic/x-pack-elasticsearch@a6e537cffe

elasticsearch-transport-client/build.gradle

@@ -31,3 +31,11 @@ namingConventions {
     //we don't have integration tests
     skipIntegTestInDisguise = true
 }
+
+publishing {
+    publications {
+        nebula(MavenPublication) {
+            pom.projectIdentity.artifactId = archivesBaseName
+        }
+    }
+}