Fix support for multiple .networks.cloudNAT.natIPNames

[ialidzhikov]

/kind bug
/platform gcp

Before this PR, the rendered outputs for multiple natIPNames was:

output "nat_ips" {
  value = data.google_compute_address.manualnat4.address,data.google_compute_address.manualnat5.address
}

which results in the error described in #265.

With this PR, it is:

output "nat_ips" {
  value = "${data.google_compute_address.manualnat4.address},${data.google_compute_address.manualnat5.address}"
}

And the corresponding outputs in the terraform.tfstate:

        "nat_ips": {
          "value": "104.199.15.99,35.205.9.207",
          "type": "string"
        },

Fixes #265

Release note:

An issue causing Infrastructure with multiple `.networks.cloudNAT.natIPNames` to fail to be reconciled is now fixed.

[ialidzhikov]

/test

[testmachinery]

Testrun: e2e-dp7t8
Workflow: e2e-dp7t8-wf
Phase: Succeeded

+---------------------+---------------------+-----------+----------+
|        NAME         |        STEP         |   PHASE   | DURATION |
+---------------------+---------------------+-----------+----------+
| infrastructure-test | infrastructure-test | Succeeded | 9m7s     |
+---------------------+---------------------+-----------+----------+

[DockToFuture]

/lgtm