Adding Debian as a Tier 2 platform

@nuclearsandwich
We prepared the buildfarm using virtual machines, and create a job using https://github.com/ros2/ci, but I have one question.

The sub-module https://github.com/osrf/rticonnextdds-src/ included in https://github.com/ros2/ci seems to be private.
Now, comment out the part of the Dockerfile that adds the license and run the job with the option not to use Connext. Do we have to contract and prepare our own RTI Connext DDS license?