[2.8] Limit number of assigned vGPUs to 1 for each Machine Pool

[torchiaf]

Summary

Fixes #10992
Fixes #10993

Occurred changes and/or fixed issues

Technical notes summary

• It will limit the number of assigned vGPUs to 1 to prevent allocation issues.
• Label format is revisited.

We will remove the constraint once Harvester will support multiple vGPUs in Rancher provisioning.

Areas or cases that should be tested

Areas which could experience regressions

Screenshot/Video

See #10991

Checklist

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[rak-phillip]

LGTM

[rak-phillip]

NOTE: I think it's a little redundant to have a computed prop. Without it, the template would be just as simple and declarative. We already employ this pattern for showAdd and showRemove, so it makes sense to stick with the prevalent pattern.