docker_swarm inventory: fix tests, and make sure that they fail if no host is found (#54380)

* Fix tests, and make sure that they fail if no host is found.

* Fix interpreter.

test/integration/targets/inventory_docker_swarm/inventory_1.docker_swarm.yml

@@ -1,3 +1,3 @@
 ---
 plugin: docker_swarm
-host: unix://var/run/docker.sock
+docker_host: unix://var/run/docker.sock

test/integration/targets/inventory_docker_swarm/inventory_2.docker_swarm.yml

@@ -1,5 +1,5 @@
 ---
 plugin: docker_swarm
-host: unix://var/run/docker.sock
+docker_host: unix://var/run/docker.sock
 verbose_output: no
 include_host_uri: yes

test/integration/targets/inventory_docker_swarm/playbooks/test_inventory_1.yml

@@ -1,6 +1,26 @@
 ---
+- hosts: 127.0.0.1
+  connection: local  # otherwise Ansible will complain that it cannot connect via ssh to 127.0.0.1:22
+  gather_facts: no
+  tasks:
+    - name: Show all groups
+      debug:
+        var: groups
+    - name: Make sure docker_swarm groups are there
+      assert:
+        that:
+          - groups.all | length > 0
+          - groups.leader | length == 1
+          - groups.manager | length > 0
+          - groups.worker | length >= 0
+          - groups.nonleaders | length >= 0
+
 - hosts: all
   connection: local  # otherwise Ansible will complain that it cannot connect via ssh to 127.0.0.1:22
+  vars:
+    # for some reason, Ansible can't find the Python interpreter when connecting to the nodes,
+    # which is in fact just localhost in disguise. That's why we use ansible_playbook_python.
+    ansible_python_interpreter: "{{ ansible_playbook_python }}"
   tasks:
     - name: Check for groups
       assert:

test/integration/targets/inventory_docker_swarm/playbooks/test_inventory_2.yml

@@ -1,6 +1,26 @@
 ---
+- hosts: 127.0.0.1
+  connection: local  # otherwise Ansible will complain that it cannot connect via ssh to 127.0.0.1:22
+  gather_facts: no
+  tasks:
+    - name: Show all groups
+      debug:
+        var: groups
+    - name: Make sure docker_swarm groups are there
+      assert:
+        that:
+          - groups.all | length > 0
+          - groups.leader | length == 1
+          - groups.manager | length > 0
+          - groups.worker | length >= 0
+          - groups.nonleaders | length >= 0
+
 - hosts: all
   connection: local  # otherwise Ansible will complain that it cannot connect via ssh to 127.0.0.1:22
+  vars:
+    # for some reason, Ansible can't find the Python interpreter when connecting to the nodes,
+    # which is in fact just localhost in disguise. That's why we use ansible_playbook_python.
+    ansible_python_interpreter: "{{ ansible_playbook_python }}"
   tasks:
     - name: Make sure docker_swarm_node_attributes is not available
       assert: