HDFS-12889. Router UI is missing robots.txt file. Contributed by Bharat Viswanadham.

hadoop-hdfs-project/hadoop-hdfs/pom.xml

@@ -389,6 +389,7 @@ http://maven.apache.org/xsd/maven-4.0.0.xsd">
             <exclude>src/main/webapps/hdfs/robots.txt</exclude>
             <exclude>src/main/webapps/journal/robots.txt</exclude>
             <exclude>src/main/webapps/secondary/robots.txt</exclude>
+            <exclude>src/main/webapps/router/robots.txt</exclude>
             <exclude>src/contrib/**</exclude>
             <exclude>src/site/resources/images/*</exclude>
             <exclude>src/main/webapps/static/bootstrap-3.0.2/**</exclude>