Compression-related changes to HiBench YARN branch

We noticed performance discrepancy between Master and YARN branch. After lot of experimentation and analysis, we found that the primary problem was compression options. Contributing some of the changes back upstream. For example:  snappyCodec should be used for map.output.compress instead of defaultCodec; by default compression should be turned off, etc. 
why we need to remove `-Dmapreduce.map.output.compress=false` here?