include_tasks と import_tasks

ansible_include_tasks_and_import_tasks_test.txt

$ ANSIBLE_STDOUT_CALLBACK=default ansible-playbook test.yml
 [WARNING]: provided hosts list is empty, only localhost is available. Note that the implicit localhost does not match 'all'


PLAY [localhost] *****************************************************************************************************************************************************************************

TASK [include_tasks] *************************************************************************************************************************************************************************
included: /home/taiki/src/github.com/smartoperation/cobit/infra/aws-playbook/test2.yml for localhost

TASK [debug] *********************************************************************************************************************************************************************************
ok: [localhost] => {
    "msg": "import_tasks は変数の上書きはできない"
}

TASK [show arg and arg2] *********************************************************************************************************************************************************************
ok: [localhost] => {
    "msg": "arg=hoge, arg2=this"
}

TASK [debug] *********************************************************************************************************************************************************************************
ok: [localhost] => {
    "msg": "import_tasks は変数の上書きはできないからこそ、二重評価もされない"
}

TASK [show arg and arg2] *********************************************************************************************************************************************************************
ok: [localhost] => {
    "msg": "arg=hoge, arg2=that"
}

TASK [debug] *********************************************************************************************************************************************************************************
ok: [localhost] => {
    "msg": "include_tasks なら変数の上書きができる"
}

TASK [include_tasks] *************************************************************************************************************************************************************************
included: /home/taiki/src/github.com/smartoperation/cobit/infra/aws-playbook/test3.yml for localhost

TASK [show arg and arg2] *********************************************************************************************************************************************************************
ok: [localhost] => {
    "msg": "arg=what?, arg2=is"
}

TASK [debug] *********************************************************************************************************************************************************************************
ok: [localhost] => {
    "msg": "include_tasks で同じ変数を使うと recursive loop detected in template string でエラーになる"
}

PLAY RECAP ***********************************************************************************************************************************************************************************
localhost                  : ok=9    changed=0    unreachable=0    failed=0   

test.yml

- hosts: localhost
  connection: local
  gather_facts: False
  tasks:
    - include_tasks: test2.yml
      vars:
        arg: hoge

test2.yml

- debug:
    msg: import_tasks は変数の上書きはできない
- import_tasks: test3.yml
  vars:
    arg: what?
    arg2: this

- debug:
    msg: import_tasks は変数の上書きはできないからこそ、二重評価もされない
- import_tasks: test3.yml
  vars:
    arg: "{{ arg }}"
    arg2: that

- debug:
    msg: include_tasks なら変数の上書きができる
- include_tasks: test3.yml
  vars:
    arg: what?
    arg2: is

- debug:
    msg: include_tasks で同じ変数を使うと recursive loop detected in template string でエラーになる
# - include_tasks: test3.yml
#   vars:
#     arg: "{{ arg }}"

test3.yml

- name: show arg and arg2
  debug:
    msg: "arg={{ arg }}, arg2={{ arg2 }}"

基本的に import_tasks は変数を指定しないで使うのがよいのだと思う。静的に解決するという意味でも。
include_tasks で使う変数は可能なら role と同様に prefix 付きにするのが良いのでしょう。