Support for OpenStack's clouds.yaml client configuration

[viq]

🙍 Problem Statement

OpenStack allows for the configuration to be specified in files, per https://docs.openstack.org/python-openstackclient/latest/configuration/index.html
It would be nice if kitchen-openstack was able to use the same configuration, instead of having to hardcode the settings in kitchen.yml

❔ Possible Solution

test-kitchen uses OpenStack credentials stored in clouds.yml file.

⤴️ Describe alternatives you've considered

ggiamarchi/vagrant-openstack-provider#350 shows a way to work around it, but I don't think test-kitchen provides "enough ruby" to do that.

➕ Additional context

I'm not sure fog supports it in the first place...