[test-operator] Improve variables overwriting

As part of processing test-operator stages, there is a task
that takes all defined Ansible variables and overrides them
with matching stage-specific variables. However, this task
focuses in the end only on variables specific to test-operator.
Yet, because the filtering is done on Ansible task-level,
we end up with 80% of the `ansible-post-deployment.log`
filled with entries like `skipping <variable_name>`.
To make this worse, it exposes some deployment-time secrets
as a side-effect.

This commit moves the filtering from task-level to earlier
phase ~ when the loop arguments are prepared, so we will
do override only on the test-operator-related variables,
reducing the useless output and avoiding secrets exposure.

Author: sdatko

roles/test_operator/tasks/stages.yml

@@ -30,12 +30,9 @@
     _stage_test_vars: "{{ file_vars | combine(test_vars) }}"
 
 - name: Overwrite global_vars with stage_vars
-  with_dict: "{{ vars | combine(_stage_test_vars) }}"
-  vars:
-    start_with: cifmw_test_operator_
-  when: item.key.startswith(start_with)
   ansible.builtin.set_fact:
     stage_vars_dict: "{{ stage_vars_dict | combine({item.key: _stage_test_vars[item.key] | default(lookup('vars', item.key, default=omit)) }) }}"
+  with_dict: "{{ vars | combine(_stage_test_vars) | dict2items | selectattr('key', 'match', '^cifmw_test_operator_') | items2dict }}"
 
 - name: Override specific type config
   vars: