[BUG] GCE metadata grain code is overly complex

[jonbenning]

Description
The gce metadata grain code was likely heavily influenced by the AWS EC2 grains, and utilizes complex recursion logic that is not needed in GCP.

Setup
There is nothing presently broken in the current metadata_gce.py code, but its difficult to read, and could be far simpler.

Please be as specific as possible and give set-up details.

• on-prem machine
• VM (Virtualbox, KVM, etc. please specify)
• VM running on a cloud service, please be explicit and add details
• container (Kubernetes, Docker, containerd, etc. please specify)
• or a combination, please be explicit
• jails if it is FreeBSD
• classic packaging
• onedir packaging
• used bootstrap to install

Steps to Reproduce the behavior
(Include debug logs if possible and relevant)

Expected behavior
Code should retrieve all available (non-sensitive) metadata from the gcp metadata service and create grains accordingly

Screenshots
If applicable, add screenshots to help explain your problem.

Additional context

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