test: adapt Molecule setup to suit peculiarities from the latest Molecule v6 release

Molecule now runs from a different directory than it used to (Change current directory to scenario before running playbooks), so it doesn't find Ansible Vault password file in the same relative path anymore. The changes made here fix that for us.

We've seized the opportunity to also create a firewall rule to allow access from Molecule host into testing VM, so we don't get locked out by our own Ansible tasks and retain our ability to run molecule idempotence.