provider/google: fix container instance group URLs

[paddycarver]

Google Container Engine's cluster API returned instance group manager
URLs when it meant to return instance group URLs. See #4336 for details
about the bug.

While this is undeniably an upstream problem, this PR:

• detects the error, meaning it will work as expected when the API is
  fixed.
• corrects the error by requesting the instance group manager, then
  retrieving its instance group URL, and using that instead.
• adds a test that exercises the error and the solution, to ensure it is
  functioning properly.

Unit test without fix:

=== RUN   TestAccContainerCluster_backend
--- FAIL: TestAccContainerCluster_backend (303.57s)
        testing.go:265: Step 0 error: Error applying: 1 error(s) occurred:

                * google_compute_backend_service.my-backend-service: 1 error(s) occurred:

                * google_compute_backend_service.my-backend-service: Error creating backend service: googleapi: Error 400: Invalid value for field 'resource.backends[0].group': 'https://www.googleapis.com/compute/v1/projects/hc-terraform-testing/zones/us-central1-c/instanceGroupManagers/gke-terraform-test-evnwj-default-pool-68b6a1a7-grp'. The value is not a valid resourceView nor instanceGroup URL., invalid
FAIL

Unit test with fix:

=== RUN   TestAccContainerCluster_backend
--- PASS: TestAccContainerCluster_backend (298.37s)

[lwander]

Nice, LGTM. Let's leave this open briefly before merging to see if anyone else from that open bug wants to weigh in.

[paddycarver]

Going to go ahead and merge this one now. :)

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

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