Define Maven plugin repository for gs-maven-wagon artifact

Without having the plugin repository specified Maven is unable to find
and download the gs-maven-wagon artifact and the build fails.

Change-Id: If726f0bdcfa94449cd1427c78d2c734e175ce99b
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>
diff --git a/pom.xml b/pom.xml
index 27a3d3a..74e6fea 100644
--- a/pom.xml
+++ b/pom.xml
@@ -360,6 +360,13 @@
     </dependency>
   </dependencies>
 
+  <pluginRepositories>
+    <pluginRepository>
+      <id>gerrit-maven</id>
+      <url>https://gerrit-maven.commondatastorage.googleapis.com</url>
+    </pluginRepository>
+  </pluginRepositories>
+
   <distributionManagement>
     <repository>
       <id>gerrit-maven-repository</id>