benchmarks: make ensure_cpu_scaling more resilient
benchmarks: make ensure_cpu_scaling more resilient

It's called out of a trap, and since the script uses pushd/popd a lot, it's
not always reachable:

  + python write-jobs-stats-to-mysql.py kudu-benchmarks 1039 DenseNodeItest_time_bootstrapping_tablets 8
  usage: write-jobs-stats-to-mysql.py <job_name> <build_number> <workload> <iteration> <runtime>
  + restore_governor
  + ensure_cpu_scaling performance
  ++ dirname ./src/kudu/scripts/benchmarks.sh
  + ./src/kudu/scripts/ensure_cpu_scaling.sh performance
  ./src/kudu/scripts/benchmarks.sh: line 93: ./src/kudu/scripts/ensure_cpu_scaling.sh: No such file or directory

Change-Id: I3452b48577dd8f5f72c57d41ccf41848c682d616
Reviewed-on: http://gerrit.cloudera.org:8080/6847
Tested-by: Adar Dembo <866ca730bfc7f544e3ebce604ceeba4879f23df0@cloudera.com>
Tested-by: Kudu Jenkins
Reviewed-by: David Ribeiro Alves <dbbafdb4f25eb0c1ff3facf0e5f2f27705055af1@gmail.com>

InspurUSA/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,cloudera/kudu,EvilMcJerkface/kudu,helifu/kudu,andrwng/kudu,InspurUSA/kudu,InspurUSA/kudu,InspurUSA/kudu,helifu/kudu,andrwng/kudu,InspurUSA/kudu,cloudera/kudu,helifu/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,andrwng/kudu,helifu/kudu,EvilMcJerkface/kudu,cloudera/kudu,andrwng/kudu,andrwng/kudu,helifu/kudu,andrwng/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,helifu/kudu,InspurUSA/kudu,helifu/kudu,EvilMcJerkface/kudu,cloudera/kudu,cloudera/kudu,EvilMcJerkface/kudu,cloudera/kudu,helifu/kudu,cloudera/kudu,andrwng/kudu,helifu/kudu,andrwng/kudu,andrwng/kudu,cloudera/kudu,cloudera/kudu