Try Docker as Beaker provider

[dfarrell07]

Most people seem to use Vagrant as the provider Beaker uses for standing up test VMs. I have that setup working well, but it's quite slow. Docker would allow caching of unchanged steps, dramatically speeding up the process.

This is a long-term TODO.

[dfarrell07]

Using the Beaker workflow described in the numbered points here dramatically speeds up testing, making this less critical.

[dfarrell07]

Vagrant Beaker tests have been historically unstable and are currently quite broken on my system. It's also not easy/possible to run VM-based tests on ODL'c CI VMs (nested virt issues), so moving containers is desirable for that future use-case. For both of these reasons, I'm bumping the prio of this TODO and am actively working on it.