Fix pre_test_hook in rally-scenarios-job

pre_test_hook.sh should be run in the current shell context. This is
mandatory because we need to export some environment variables.

Also fixed `voting` line for this job.

Change-Id: I49632bd323ad7abdcfaff5d8295a4557fa8e34ec
Merge "Fix pre_test_hook in rally-scenarios-job"