.travis.yml: switch to orocos/ros2-ci Docker images without preinstalled rtt_ros2_integration for CI

[meyerj]

Same as orocos/rtt_ros2_integration#26 for rtt_ros2_integration:

Once orocos/orocos-docker-images#3 has been merged and the issues with automated builds on Docker Hub have been solved, the orocos/ros2 images will have rtt_ros2_integration and rtt_ros2_common_interfaces preinstalled. This might be bad for CI testing of those two repos, if older versions of the packages to be tested are preinstalled in an underlay.

Therefore a new Docker repository orocos/ros2-ci has been created, whose images are equivalent to the ones in orocos/ros2 (after orocos/orocos-docker-images#3) but stop at the orocos_toolchain target and hence only have the Orocos Toolchain packages preinstalled.

A fundamental difference compared to the previous images is that the Orocos Toolchain and ROS integration packages are installed to /opt/orocos/${ROS_DISTRO} now and not to /opt/ros/${ROS_DISTRO}. The main purpose of this pull request is to verify whether industrial_ci can deal with that. industrial_ci/src/tests/source_tests.sh only extends /opt/ros/${ROS_DISTRO} and there is no configuration variable to change that, but /opt/orocos/${ROS_DISTRO} is already sourced in the orocos_entrypoint.sh script and hence the required variables should be already set (CMAKE_PREFIX_PATH, LD_LIBRARY_PATH, OROCOS_TARGET, ...).

The current set of images in orocos/ros2-ci have been built and pushed manually because automated builds for orocos/orocos-docker-images#3 on Docker Hub still fail, likely because of resource constraints.