Support for releasing ros_gzgarden unofficial packages for Iron

[bperseghetti]

Motivation

Same as #895 but for Iron. @azeey @j-rivero

Roadmap (edited by @j-rivero):

• Support for rosdep to rename Gz Fortress to Gz Garden
  • Support for Iron renaming of Fortress to Garden osrf/osrf-rosdep#14
• Parametrize tooling in release-tools to support different ROS 2 versions
  • Parametrize ros_gz bloom tooling to accept different ROS_DISTRO values #958
• Release a test version into the prerelease repository using fork -release repo
• Use j-rivero fork to generate metadata and to create the prerelease from it
• Test in the buildfarm using the manual-any job
  • 
• Create the track for iron in the ros_ign-release (can be autogenerated by bloom?)
• Push info to gazebo-release/ros_ign-release repository
• Release into stable from the ros_ign-release repository
• DSL support for testing Iron GzGarden
  • Support for ROS 2 Iron CI in gzgarden wrappers #960

Testing:

Install the Gazebo prerelease repository
apt-get update && apt-get install ros-iron-ros-gzgarden (will uninstall Iron official packages if they are installed)

[j-rivero]

Iron packages are in the prerelease repository. Ready for testing.

[bperseghetti]

Should be able to test it starting this week!

[j-rivero]

Stable release is out.