Fixed: workload_runner fails when there is an underscore in workload name

Change-Id: Ic18c5409f87c14b4b3f80baef1302a1d9cff3bc9
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/3745
Reviewed-by: Taras Bobrovytsky <tbobrovytsky@cloudera.com>
Tested-by: jenkins