[horizon] Changing the release model to cycle-with-intermediary

Hi,

I just proposed a patch to change the horizon release model to cycle-with-intermediary. https://review.opendev.org/#/c/694772/

It was discussed during the Shanghai PTG. Horizon provides GUI to users. On the other hand, it is a library for horizon plugins. When horizon plugins would like to use recent changes or to avoid bugs, they need to consume beta releases of horizon. More frequent releases of horizon would make more sense, so I am proposing the release model change.

If there are concerns, reply to this thread or drop comments in the review mentioned above.

Thanks, Akihiro Motoki (irc: amotoki)