Add support for compute network routing mode

[mrparkers]

I had a need for this when building a VPC that talks to an on-prem network, and noticed that this provider doesn't have a way to specify the VPC routing mode. The current workaround is to add gcloud compute networks update my-vpc --project my-project --bgp-routing-mode global to local-exec after creating the VPC.

I did a quick search and found that this was asked for in #629.

Docs:

• Rest API: https://cloud.google.com/compute/docs/reference/latest/networks (refer to routingConfig)
• Cloud Router docs: https://cloud.google.com/router/docs/concepts/overview#dynamic-routing-mode

Tests:

TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run=TestAccComputeNetwork.*routing_mode -timeout 120m
?   	github.com/terraform-providers/terraform-provider-google	[no test files]
=== RUN   TestAccComputeNetwork_routing_mode
=== RUN   TestAccComputeNetwork_default_routing_mode
--- PASS: TestAccComputeNetwork_routing_mode (336.44s)
--- PASS: TestAccComputeNetwork_default_routing_mode (336.46s)
PASS
ok  	github.com/terraform-providers/terraform-provider-google/google	336.487s
testing: warning: no tests to run
PASS
ok  	github.com/terraform-providers/terraform-provider-google/scripts	0.017s [no tests to run]

Side note: It is possible to change the routing mode after creating the VPC, but this resource doesn't have an UpdateFunc and I didn't want to go through the work of supporting it in this PR. However, if you'd like me to add it before this gets merged, I can do that.

[terraform-ctc]

Great work! We have the same use case, vpc w/ on-prem networks, so would love to see this get merged asap.

[rosbo]

Thank you Michael for adding this feature! :)

I filed a separate issue (#846) for adding update support for this field.

[mrparkers]

#847 you mean? 😉

[rosbo]

Ooopsi. Yes :)

[ghost]

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