jenkins: all test suite runs on base branches are considered failures

(Just in case it isn't clear outside of FT, base branches are considered stable, testing and devel, but until puppet-tails!140 (merged) is merged this actually affects all branches that doesn't start with a number.)

On jenkins,

• test_Tails_ISO_stable #4565 from Dec 12 was the last run where all scenarios passed and the job was correclty marked as a success, and
• test_Tails_ISO_stable #4576 from Dec 18 is (AFAICT) the first run after that where all scenario passed but the job was incorrectly considered a failure.

So what happened between Dec 12 and 18? On Dec 14 puppet-tails@ba5ba04f started running sys.exit("...") for these branches, and it will exit with 1 (so error) when passed a string. I am not 100% sure this is the cause, but it looks promising. The fix would be to just print() the message and then sys.exit(0). I added this potential fix to my puppet-tails!140 (merged) MR.