Add CDH_MAJOR_VERSION environment variable

CDH_MAJOR_VERSION controls where HDFS data is written. In the future, we
can use its value to parameterise Jenkins jobs so that the right code is
run / data is generated.

Change-Id: Id2957df6d708bc6c50faf7a8a609aff5f9571662
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1293
Reviewed-by: Nong Li <nong@cloudera.com>
Reviewed-by: Lenni Kuff <lskuff@cloudera.com>
Tested-by: jenkins