Fix passwords.yml generation with vault encryption on Python 3

Kayobe generates passwords.yml for Kolla Ansible, and can encrypt it
using the vault password. Previously this was failing on Python 3 due to
passing a string to file.write() which expects bytes.

This change fixes the issue by encoding the password string passed to
file.write().

This allows us to run the ansible role tests under Python 3.

Change-Id: I33813f79984a46f1967ef3aee455dcfbe7eb93da
Story: 2006574
Task: 39481