HDFS-9651. Add missing robots.txt files.

(cherry picked from commit 8cb73255274bd46ea8aedbb28f25fa13b38723ad)

hadoop-hdfs-project/hadoop-hdfs/src/main/webapps/hdfs/robots.txt

@@ -0,0 +1,2 @@
+User-agent: *
+Disallow: /

hadoop-hdfs-project/hadoop-hdfs/src/main/webapps/journal/robots.txt

@@ -0,0 +1,2 @@
+User-agent: *
+Disallow: /

hadoop-hdfs-project/hadoop-hdfs/src/main/webapps/secondary/robots.txt

@@ -0,0 +1,2 @@
+User-agent: *
+Disallow: /