[Bug]: `metadata-service` uses inconsistent config location `$HOME/.metadata-service` instead of `$HOME/.config/metadata-service`

[seantronsen]

Describe the bug

boot-service uses the latter as of OpenCHAMI/boot-service#40, thus rendering this behavior inconsistent.

For simplicity, we should default to XDG_CONFIG_HOME. go provides a func in the stdlib already for this: os.UserConfigDir

Steps to reproduce

See 02a3722

Relevant log output

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