base and core images must come from official docker library

[ijnek]

Currently, this template generates a Dockerfile from osrf/ros:rolling-ros-base, but it really should be basing on ros:rolling-ros-base (without osrf/)

Related: osrf/docker_images#672

[ijnek]

I've removed these from the templates for now, as they just don't work. (And it's actually a bit tricky to get them working)