Finish GCLB Terraform setup

[icco]

Proposed Changes

• Waiting on next version of google-beta, which will have Max utilization is not supported for Serverless network endpoint groups. hashicorp/terraform-provider-google#7051
• Attempted testing using https://github.com/hashicorp/terraform-provider-google/blob/master/.github/CONTRIBUTING.md#running-a-local-copy-of-the-provider, but couldn't quite get it working, so putting up here for now, and will return to in bit.

Release Note

Adds three new host variables to terraform and sets up a GCLB with those hosts.

[google-oss-robot]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[icco]

/assign @sethvargo

[sethvargo]

/lgtm
/approve

[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: icco, sethvargo

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