Add feature to expose vars/defaults with include/import_role (#41330)

* First pass at making 'private' work on include_role, imports are always public

* Prevent dupe task execution and overwriting handlers

* New functionality will use public instead of deprecated private

* Add tests for public exposure

* Validate vars before import/include to ensure they don't expose too early

* Add porting guide docs about public argument and change to import_role

* Add additional docs about public and vars exposure to module docs

* Insert role handlers at parse time, exposing them globally

lib/ansible/modules/utilities/logic/include_role.py

@@ -55,8 +55,15 @@ options:
     description:
       - This option is a no op, and the functionality described in previous versions was not implemented. This
         option will be removed in Ansible v2.8.
+  public:
+    description:
+      - This option dictates whether the role's C(vars) and C(defaults) are exposed to the playbook. If set to C(yes)
+        the variables will be available to tasks following the C(include_role) task. This functionality differs from
+        standard variable exposure for roles listed under the C(roles) header or C(import_role) as they are exposed at
+        playbook parsing time, and available to earlier roles and tasks as well.
     type: bool
     default: 'no'
+    version_added: '2.7'
 notes:
   - Handlers are made available to the whole play.
   - Before Ansible 2.4, as with C(include), this task could be static or dynamic, If static, it implied that it won't