Separate ros2-docs distro param and ticket label param

[Yadunund]

Right now the YATM_DISTO_LABEL envar is used for both

• As the ROS distro version whose docs.ros.org sitemap needs to be combed for generating tutorial requirements and
• A github label that should be appended to any github tickets opened

We should separate the two. Potential solution proposed here #368 (comment). Perhaps we can also get rid of YATM_DISTO_LABEL have a CLI argument for the github ticket label as well.