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 command for specifying the HMC and credentials from these inventory and vault files, by referencing the HMC nickname instead of the combination of -n, -h, -u, -p.