packer: add machine_type configuration

[ledmonster]

I added machine_type option, because I couldn't use default machine_type, "n1-standard-1" with asia-northeast1-(a, b, c) zone.

I also made PR to change default machine_type.

• https://github.com/hashicorp/packer-plugin-googlecompute/pull/171

[github-actions[bot]]

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