Add new HubType = Docker and necessary CapabilityProviders

[art-cmyk]

Add a new HubType = Docker which can run Chrome and Firefox tests on Linux (and hopefully Windows soon with Windows 2016 Docker containers + Edge as well). This will allow the tester to request a configuration and run the tests locally without the task to setting up Hub and Node locally. Another advantage is that users can run tests on Linux without an actual Linux VM.