Ansible doesn't seem to correctly configure python interpreter
while running in a virtual environment and defaults to the system
wide installation. This can lead to missing dependencies and/or
version mismatch issues. This change adds automatic configuration
of ansible_python_interpreter value based on VENV environment
variable for each of the roles provided by bifrost.
Change-Id: I0bc6a0ee7519bca0fe81eb74349427681269a740
3c5377ce92