Support clouds.yaml / application credentials

[jkrue]

The current version of pKA use environment variables (OpenRC File) for authentication. Since Openstack-SDK also supports clouds.yaml and applications credentials since a decade we should also support them.

[jkrue]

Clouds.yaml is partly supported by current implementations. keystone.py could be simplified using sdk methods.

import os_client_config

sdk = os_client_config.make_sdk(cloud="master")