This can potentially cause security issues.

Ubuntu 16.04 Docker images don't come with sudo.

Ubuntu Trusty's Python 3 package supplies 3.4,
so this makes it possibly to run the tests on Trusty machines
without manually installing Python 3.5.

To allow running inside Docker,
create our own virtualenv for Python test dependencies (e.g. `jinja2`).
Don't use the virtualenvs supplied by Travis.
Make sure to install some extra Python bits
that aren't installed by default for Docker.

Rewrite testing code to avoid APIs introduced in Python 3.5.
Use block indent and early return in tests.

More work is needed to support the cross build states.

`service.running` requires talking to init,
which is not running inside Docker.

`timezone.system` on systemd distros uses timedatectl
which also wants to talk to a running systemd,
so skip that on Docker as well.

Use pinned Docker images for 14.04 and 16.04.
Re-exec the dispatch script inside a running container
to simulate running the tests on other OSes.

This is not perfect since various states do
not function properly in containers
(mainly service-related states),
but is still better than nothing
and tests package installation, etc.