Variable substitution in task meta data

Hi list,

my goal: A role which allows to configure various aspects how tasks
should behave, e.g. how to use the become subsystem, especially for
local_action tasks.
Ultimate goal: Configure a few variables in inventory, and have all
roles behavior configured for that specific infrastructure.

# Assumptions:

- Role foo depends on role foobar (meta/main.yml dependencies:)
- Role bar depends on role foobar (meta/main.yml ...)
- Role foobar provides variables to control the become subsystem.
- Role foo and bar use variables from foobar.

# Role foobar

## defaults/main.yml

    foobar_action_become_enable: yes
    #foobar_action_become_user: root
    #foobar_action_become_method: sudo

# Role foo

## tasks/main.yml

    - name: Do something that requires sudo
      become: "{{ foobar_action_become_enable }}"
      become_user: "{{ foobar_action_become_user|default(omit) }}"
      become_method: "{{ foobar_action_become_method|default(omit) }}"
      file:
        state=directory
        dest=/usr/local/foo
        owner=0
        group=0
        mode=0755

I did something similar with
[groover.util](https://github.com/silpion/ansible-util/tree/0.1.0) role
regarding sudo and sudo_user, which felt to work as intended.

Thing is, ... this doesn't work (anymore, or at least with become).

# Example playbook