[feature]new plugin googlecompute machine image

I would like to contribute to this repo by creating an machine image gcp resource, basically is a clone of `googlecompute` but instead of saving as another gcp image, it will be saved as `machine image`.

https://cloud.google.com/compute/docs/machine-images

Should i start creating another builder called `googlecomputemachineimage`?

The process will be basically the following:

1. Use the CreateImage part of the code to create a google compute instance and run provisioners.
2. Create the machine image based on the running google compute instance.
3. Delete the running instance.

The other option would be to extend the current `googlecompute` builder and add a conditional to create the machine image `create_machine_image` and the `steps` will basically create a new machine image and teardown the instance afterwards without creating a real gcp image.