Support for subnetwork on google_container_cluster resource

[devth]

https://cloud.google.com/sdk/gcloud/reference/container/clusters/create

[devth]

Currently if a Network is in custom mode, it's impossible to create a cluster since Subnetwork is a required parameter in that case:

* google_container_cluster.standard-test-public: googleapi: Error 400: Subnetwork must be provided for manual-subnet Network "test-m"., badRequest

[lwander]

We try to avoid adding support for non-GA features, a good explanation is given here: #5053 (comment).

[devth]

The self_link issue may not apply here. For example, in #5552 I was able to work around by specifying a string name instead of relying on self_link.

[lwander]

The issue is that if anyone has an existing resource that refers to a cluster with something like ${google_container_cluster.name.self_link}, that reference's value will change and include alpha. The resource referring to the cluster may then have to be destroyed and recreated, which is undesirable.

[JorritSalverda]

I don't think subnetwork is in beta, seeing it's part of stable gcloud:

https://cloud.google.com/sdk/gcloud/reference/container/clusters/create

I get the following error when trying to create a google_container_cluster in a project with manually configured subnets:

* google_container_cluster.staging: googleapi: Error 400: Subnetwork must be provided for manual-subnet Network "staging"., badRequest

[JorritSalverda]

Sorry, a closer look shows me it's an alpha feature even though it's in stable gcloud command.

[devth]

Fixed by #5871

[ghost]

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