We currently iterate over every job/host/etc variable in the freeze
playbook. The reason is because if any value in any variable is
Undefined according to jinja, the Ansible combine filter throws
an error. What we want to do in Zuul is merge any variable we can,
but if any is undefined, we skip it. Thus, the process of combining
the variables one at a time in a task and ignoring errors.
This process can be slow, especially if we have start with a large
amount of data in one of the early variables. The combine filter
needs to reprocess the large data repeatedly for each additional
variable.
To improve the process, we create a new action plugin, "zuul_freeze"
which takes a list of variables we want to freeze, then templates
them one at a time and stores the result in a cacheable fact. This
is the essence of what we were trying to accomplish with the combine
filter.
Change-Id: Ie41f404762daa1b1a5ae47f6ec1aa1954ad36a39
e55748ba69