Add sweeper for Compute VM Instances (google_compute_instance)

[rileykarson]

VM Instances aren't generated, and would be a good target for a handwritten sweeping function. We create a lot of them, so they're often left behind after test runs. They keep an entire network & it's associated routes, as networks can't be deleted if they have users.

We can probably skip instances managed by an igm/rigm, if that's easy to find.

Also we should prefix them all with tf-test.

[ghost]

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