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/default/molecule.yml

@@ -53,8 +53,6 @@ provisioner:
   inventory:
     links:
       group_vars: ../../inventory/sample/group_vars
-  env:
-    OVERRIDES_FILE: ../default/overrides.yml
 scenario:
   test_sequence:
     - dependency