Update Hadoop MapReduce and HDFS scripts

* The script that detects DataNodes, `check_hadoop_datanodes.rb`, was using a deprecated command `hadoop dfsadmin -report`, which was replaced with the new one.
* The script that detects TaskTrackers, `check_hadoop_tasktracker.sh`, contained a bug. The parameters represent the number of nodes that need to be available for critical and warning state. Thus, there must be **more** nodes in warning state than in critical state, not less as it was previously. Additionally, the order of the `if`s from the condition was wrong preventing any warning to be reported.