docker_config and docker_secret: add data_src option (#203)

* Add data_src options to docker_config and docker_secret.

* Add examples.

* Fix test.

* Another fix.

tests/integration/targets/docker_secret/tasks/test_secrets.yml

@@ -33,7 +33,7 @@
     assert:
       that:
          - 'output.failed'
-         - 'output.msg == "state is present but all of the following are missing: data"'
+         - 'output.msg == "state is present but any of the following are missing: data, data_src"'
 
   - name: Create secret
     docker_secret:
@@ -76,6 +76,24 @@
       that:
          - not output.changed
 
+  - name: Write secret into file
+    copy:
+      dest: "{{ remote_tmp_dir }}/data"
+      content: |-
+        opensesame!
+
+  - name: Create secret again (from file)
+    docker_secret:
+      name: db_password
+      data_src: "{{ remote_tmp_dir }}/data"
+      state: present
+    register: output
+
+  - name: assert create secret is idempotent
+    assert:
+      that:
+         - not output.changed
+
   - name: Create secret again (base64)
     docker_secret:
       name: db_password