Test single-node cluster (#78)

* Molecule: Derive overrides.yml location from scenario dir

# Conflicts:
#	molecule/default/molecule.yml
#	molecule/ipv6/molecule.yml

* Molecule: Add single_node scenario

* Fix get_nodes test for the case of empty groups

molecule/resources/converge.yml

@@ -1,7 +1,7 @@
 ---
 - name: Apply overrides
   ansible.builtin.import_playbook: >-
-    {{ lookup("ansible.builtin.env", "OVERRIDES_FILE") }}
+    {{ lookup("ansible.builtin.env", "MOLECULE_SCENARIO_DIRECTORY") }}/overrides.yml
 
 - name: Converge
   ansible.builtin.import_playbook: ../../site.yml

molecule/resources/reset.yml

@@ -1,7 +1,7 @@
 ---
 - name: Apply overrides
   ansible.builtin.import_playbook: >-
-    {{ lookup("ansible.builtin.env", "OVERRIDES_FILE") }}
+    {{ lookup("ansible.builtin.env", "MOLECULE_SCENARIO_DIRECTORY") }}/overrides.yml
 
 - name: Reset
   ansible.builtin.import_playbook: ../../reset.yml

molecule/resources/verify/from_outside/tasks/test/get-nodes.yml

@@ -12,6 +12,13 @@
     fail_msg: "Expected nodes {{ expected_nodes }}, but found nodes {{ found_nodes }}"
   vars:
     found_nodes: >-
-      {{ cluster_nodes | json_query('resources[*].metadata.name') | unique }}
-    expected_nodes: >-
-      {{ (groups['master'] + groups['node']) | unique }}
+      {{ cluster_nodes | json_query('resources[*].metadata.name') | unique | sort }}
+    expected_nodes: |-
+      {{
+        (
+          ( groups['master'] | default([]) ) +
+          ( groups['node'] | default([]) )
+        )
+        | unique
+        | sort
+      }}