Use Gazebo Ionic in tutorial images (moveit#3196)

Author: sea-bass

.docker/tutorial-source/Dockerfile

@@ -4,7 +4,7 @@
 # Source build of the repos file from the tutorial site
 
 ARG ROS_DISTRO=rolling
-ARG GZ_VERSION=harmonic
+ARG GZ_VERSION=ionic
 
 FROM moveit/moveit2:${ROS_DISTRO}-ci
 LABEL maintainer Tyler Weaver [email protected]