Migrating Gazebo 7 Docker tags to Xenial base image

FYI: If you are using gazebo 7 tags from the Official Library on Docker Hub and are expecting trusty as the base image, you should know an issue has been created for migrating to xenial. We expect users who using Gazebo 7 with Trusty are doing so for ROS Indigo, and should be building from the Official ROS tags anyway.

You can find more about this from the github ticket below: