reboot_job is broken since Jenkins was upgraded to Stretch, which often breaks the test suite

Originally created by @anonym on #12259 (Redmine)

See e.g.: https://jenkins.tails.boum.org/job/test_Tails_ISO_test-12019-totem-add-local-video-action/29/console

[...]
19:27:23 Command failed (returned pid 4334 exit 255): ["/var/lib/jenkins/workspace/test_Tails_ISO_test-12019-totem-add-local-video-action/submodules/chutney/chutney", "start", "/var/lib/jenkins/workspace/test_Tails_ISO_test-12019-totem-add-local-video-action/features/chutney/test-network", {:err=>[:child, :out]}]:
19:27:23 Using Python 2.7.9
19:27:23
19:27:23 Starting nodes
19:27:23
19:27:23 Couldn't launch test000auth (tor --quiet -f /tmp/TailsToaster/chutney-data/nodes/000auth/torrc): 1
19:27:23
19:27:23 Couldn't launch test001auth (tor --quiet -f /tmp/TailsToaster/chutney-data/nodes/001auth/torrc): 1
19:27:23
19:27:23 Couldn't launch test002auth (tor --quiet -f /tmp/TailsToaster/chutney-data/nodes/002auth/torrc): 1
[... same failures for all other nodes ...]

I have seen the same failure appear for me locally when I run the test suite (and hence Chutney) and press Ctrl+C (so Chutney is not cleaned up) and remove the TMPDIR (so the PID references are lost); if I then restart the test suite there are tor instances on the expected TCP ports, and chutney fails.