Support explicit failover when automatic failover is enabled

YARN-1029 adds automatic failover support. However, users can't explicitly ask for a failover from one RM to the other without stopping the other RM. Stopping the RM until the other RM takes over and then restarting the first RM is more involving and exposes the RM-ensemble to SPOF for a longer duration.

It would be nice to allow explicit failover through yarn rmadmin -failover command.

PS: HDFS supports -failover option.