Update Roadmap page

[christophebedard]

This is a request to update the roadmap page. It hasn't really been updated in the last 6 months (last time was a bit after the Eloquent release).

I assume this might get looked at soon since the next release is in a year, which leaves more time for preparation/planning. If it's already part of the plan, then feel free to close this in favour of a better/more detailed issue.

The one concrete action which can be done now (and is the reason why I opened this) is removing this, since it has been merged ros2/design#250

Reconsider 1-to-1 mapping of ROS nodes to DDS participants,

[dirk-thomas]

Thanks for pointing it out. I created #790 to remove the completed items.