Use consistent jobPrefix for ProberTest

Use the same tag for jobs so that SystemTestBase can cleanup leftover
containers.

Related to commit 0a5b0f8e4563818541f4884e5c531ed89dcc4d6b

Fixes #263
Merge pull request #267 from spotify/dxia/cleanup-prober-test

Use consistent jobPrefix for ProberTest
Fix bug where JobStatusCommand can throw NPE

This could happen when getting host status. We first get a list of all
jobs, then the status for each job. If a job was deleted between those
two steps, we would throw a NPE when trying to process its status.

-RN
Fixed bug where 'helios status --host' would sometimes throw NPE