[tripleo] deprecating Mistral service

Hi folks,

In our long term goal to simplify TripleO and deprecate the services that aren't used by our community anymore, I propose that we deprecate Mistral services. Mistral was used on the Undercloud in the previous cycles but not anymore. While the service could be deployed on the Overcloud, we haven't seen any of our users doing it. If that would be the case, please let us know as soon as possible. Removing it from TripleO will help us with maintenance (container images, THT/puppet integration, CI, etc). Maybe we could deprecate it in Victoria and remove it in Wallaby?

Thanks,