provider/google: Set instances to computed in compute_instance_group

[stack72]

Fixes: #15024

% make testacc TEST=./builtin/providers/google TESTARGS='-run=TestAccComputeInstanceGroup_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/06/02 20:48:54 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/google -v -run=TestAccComputeInstanceGroup_ -timeout 120m
=== RUN   TestAccComputeInstanceGroup_basic
--- PASS: TestAccComputeInstanceGroup_basic (123.64s)
=== RUN   TestAccComputeInstanceGroup_update
--- PASS: TestAccComputeInstanceGroup_update (150.37s)
=== RUN   TestAccComputeInstanceGroup_outOfOrderInstances
--- PASS: TestAccComputeInstanceGroup_outOfOrderInstances (103.71s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/google	377.737s

[grubernaut]

LGTM

[ghost]

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