Shell jobs without descriptions break report generation.

[Daniel Manrique]

Even though shell jobs are supposed to run automatically, they should still all have a description, because some reporting tools may depend on that (specifically, checkbox-certification's certify_report.py plugin). When building a message based on a test, it assumes the test has a description field (line 191). It's been known to fail for some recently-introduced jobs with no description.

The onus of dealing with descriptionless jobs should potentially fall to the reporting tool, but enforcing consistency in all shell jobs so they have descriptions would also make sense. There's a place for this: checkbox/tests/message_files.py already has some tests to ensure job files and definitions make sense.