Using FQDN instead of IP to access servers with DNS resolving

With HDFS-14118, clients can get the IP of the servers (NN/Routers) and use the IP addresses to access the machine. This will fail in secure environment as Kerberos is using the domain name  (FQDN) in the principal so it won't recognize the IP addresses.

This task is mainly adding a reverse look up on the current basis and get the domain name after the IP is fetched. After that clients will still use the domain name to access the servers.