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