docker_image: allow to specify pull platform (#89)

* Allow to specify pull platform.

* Add basic test and document that the value is not used for idempotency at the moment.

* Fix pulling.

* Simplify code.

* Add API version for pull_platform.

* Move pull_platform into new pull option. Use apply_defaults=True to avoid some special logic.

* Add example.

* Remove apply_defaults=True.

tests/integration/targets/docker_image/tasks/tests/basic.yml

@@ -25,6 +25,8 @@
     name: "{{ docker_test_image_hello_world }}"
     state: present
     source: pull
+    pull:
+      platform: amd64
   register: present_1
 
 - name: Make sure image is there (idempotent)
@@ -32,6 +34,8 @@
     name: "{{ docker_test_image_hello_world }}"
     state: present
     source: pull
+    pull:
+      platform: amd64
   register: present_2
 
 - assert: