Magnum "enable_cluster_user_trust" documentation

Operating system distribution and version: CentOS Linux release 7.6.1810
Kolla-Ansible package version: kolla-ansible 8.0.1.dev77
install_type (source/binary) and distribution from /etc/kolla/globals.yml: centos source stein
Are you using official images from Docker Hub or self built? Official

enable_cluster_user_trust defaults to "false" for security reasons as setting it to "true" enables Magnum clusters to make API calls that change the owning project. Unfortunately these API calls are what make Magnum-created clusters so useful, they enable mounting Cinder volumes and creating Octavia load balancers among other things.

It has been determined by the PTL (mgoddard) in IRC that the default value of "false" should not be changed due to the aforementioned security issues. We need to document this default configurations lack of functionality in the projects documentation. It may also benefit to add a variable that controls this to globals.yml with a brief note on its implications.