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

.github/workflows/test.yml

@@ -15,6 +15,8 @@ jobs:
       matrix:
         scenario:
           - default
+          - single_node
+      fail-fast: false
     env:
       PYTHON_VERSION: "3.10"