The map output field list spec is under attribute 
 * "mapreduce.fieldsel.map.output.key.value.fields.spec".
 * The value is expected to be like "keyFieldsSpec:valueFieldsSpec"
 * key/valueFieldsSpec are comma (,) separated field spec: fieldSpec,fieldSpec,fieldSpec ...
 * Each field spec can be a simple number (e.g. 5) specifying a specific field, or a range
 * (like 2-5) to specify a range of fields, or an open range (like 3-) specifying all 
 * the fields starting from field 3. The open range field spec applies value fields only.
 * They have no effect on the key fields.
The map output field list spec is under attribute 
 * "mapreduce.fieldsel.map.output.key.value.fields.spec".
 * The value is expected to be like "keyFieldsSpec:valueFieldsSpec"
 * key/valueFieldsSpec are comma (,) separated field spec: fieldSpec,fieldSpec,fieldSpec ...
 * Each field spec can be a simple number (e.g. 5) specifying a specific field, or a range
 * (like 2-5) to specify a range of fields, or an open range (like 3-) specifying all 
 * the fields starting from field 3. The open range field spec applies value fields only.
 * They have no effect on the key fields. | FieldSelectionMapReduce.java