Allow specifying job prefix for temporary jobs

When building a TemporaryJobs, you can now specify a job prefix for all of
the jobs the TemporaryJobs creates.
Use consistent jobPrefix for helios-testing tests

When creating temporary jobs in the tests for helios-testing, ensure that
all the jobs have a consistent job prefix (i.e., `testTag`). This in turn
ensures that the jobs are cleaned up by SystemTestBase's teardown.
Merge pull request #260 from spotify/rohan/fix-tests

Ensure containers are cleaned up after helios-testing tests