Support for zhmc inventory & vault files

[andy-maier]

The zhmcclient and zhmc-ansible-modules projects have converted to use zhmc inventory and vault files. The format of these files is compatible to Ansible inventory and vault files.

Add support to the zhmc-proimetheus-exporter for getting HMC location and credentials from these inventory and vault files.

This can be done as an alternative to the existing HMC credentials file without removing that file.