[Terraform]: Restore the deprecated list splat syntax.

[rileykarson]

hashicorp/terraform#14737 deprecated the syntax we were using in a few places, but switching our tests over leads to the following error:

------- Stdout: -------
=== RUN   TestAccComputeInstanceGroup_recreatedInstances
--- FAIL: TestAccComputeInstanceGroup_recreatedInstances (0.03s)
	testing.go:538: Step 0 error: config is invalid: google_compute_instance_group.update: instances: should be a list
FAIL

This change restores the old syntax. The old syntax is going away in 0.12, so we'll need to revert this change before an update.

---

[all]

[terraform]

Restore the deprecated list splat syntax.

[terraform-beta]

[ansible]

[inspec]

[modular-magician]

I am a robot that works on MagicModules PRs!

I built this PR into one or more PRs on other repositories, and when those are closed, this PR will also be merged and closed.
depends: hashicorp/terraform-provider-google-beta#314
depends: hashicorp/terraform-provider-google#2801