[puppet] Propose retiring puppet-senlin

Hello,

I remember I raised a similar discussion recently[1] but we need the same for a different module.

puppet-selin was introduced back in 2018, but the module has had only the portion made by cookiecutter and has no capability to manage fundamental resources yet. Because we haven't seen any interest in creating implementations to support even basic usage, I'll propose retiring this module.

I'll be open for any feedback for a while, and will propose a series of patches for retirement if no concern is raised here for one week.

Thank you, Takashi

[1] http://lists.openstack.org/pipermail/openstack-discuss/2021-September/024687... [2] https://opendev.org/openstack/puppet-senlin