tests: explicitly define localhost in common-hosts

tests/ansible/hosts/common-hosts

@@ -1,5 +1,11 @@
 # vim: syntax=dosini
 
+
+# This must be defined explicitly, otherwise _create_implicit_localhost()
+# generates its own copy, which includes an ansible_python_interpreter that
+# varies according to host machine.
+localhost
+
 [connection-delegation-test]
 cd-bastion
 cd-rack11 mitogen_via=ssh-user@cd-bastion