Fix docker_plugin crash when handling plugin options (#447)

* Fix docker_plugin crash when handling plugin options.

* Try to add tests.

tests/integration/targets/docker_plugin/tasks/main.yml

@@ -0,0 +1,29 @@
+---
+# Copyright (c) Ansible Project
+# GNU General Public License v3.0+ (see LICENSES/GPL-3.0-or-later.txt or https://www.gnu.org/licenses/gpl-3.0.txt)
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+- name: Create random name prefix
+  set_fact:
+    name_prefix: "vieux/sshfs"
+    plugin_names: []
+
+- debug:
+    msg: "Using name prefix {{ name_prefix }}"
+
+- block:
+  - include_tasks: run-test.yml
+    with_fileglob:
+    - "tests/*.yml"
+
+  always:
+  - name: "Make sure plugin is removed"
+    docker_plugin:
+      plugin_name: "{{ item }}"
+      state: absent
+    with_items: "{{ plugin_names }}"
+
+  when: docker_api_version is version('1.25', '>=')
+
+- fail: msg="Too old docker / docker-py version to run docker_plugin tests!"
+  when: not(docker_api_version is version('1.25', '>=')) and (ansible_distribution != 'CentOS' or ansible_distribution_major_version|int > 6)