GCP Shared VPC installation does not log when it cannot create firewall rules

When installing OpenShift on GCP in a Shared VPC (formerly XPN) configuration, the service account used must have permissions to create firewall rules on the host project's network in order to proceed. If the account does not have permissions, the installation will fail but the explicit reason is not listed.

4.14-ec.1

100% of the time when the service account creating the cluster does not have Owner permissions or `compute.firewall.create` on the host project.

1. Follow instructions at https://docs.openshift.com/container-platform/4.13/installing/installing_gcp/installing-gcp-shared-vpc.html
2. As part of the prerequisites, make a service account with the permissions listed at https://docs.openshift.com/container-platform/4.13/installing/installing_gcp/installing-gcp-account.html#minimum-required-permissions-ipi-gcp-xpn
3. Create a cluster using an install-config.yaml similar to the one attached

The cluster fails to bootstrap. The bootstrap node will be present, as will the masters, but components will not be able to reach the api-int load balancer.

The log files would include an error message regarding the missing permissions, and possibly abort the installation early.

https://docs.openshift.com/container-platform/4.13/installing/installing_gcp/installing-gcp-account.html#minimum-required-permissions-ipi-gcp-xpn does not list the `compute.firewalls.create` permission, which is included in the code at https://github.com/openshift/installer/blob/4f59664588c4472b7aba2838159651e729908dff/pkg/asset/cluster/tfvars.go#L79.
This is probably also a related docs improvement.

additionalTrustBundlePolicy: Proxyonly
apiVersion: v1
baseDomain: installer.gcp.devcluster.openshift.com
compute:
- architecture: amd64
  hyperthreading: Enabled
  name: worker
  platform: {}
  replicas: 3
controlPlane:
  architecture: amd64
  hyperthreading: Enabled
  name: master
  platform: {}
  replicas: 3
credentialsMode: Passthrough
featureSet: TechPreviewNoUpgrade
metadata:
  creationTimestamp: null
  name: nrbxpn
networking:
  clusterNetwork:
  - cidr: 10.128.0.0/14
    hostPrefix: 23
  machineNetwork:
  - cidr: 10.0.0.0/16
  networkType: OVNKubernetes
  serviceNetwork:
  - 172.30.0.0/16
platform:
  gcp:
    projectID: openshift-installer-shared-vpc
    region: us-central1
    network: installer-shared-vpc
    computeSubnet: installer-shared-vpc-subnet-1
    controlPlaneSubnet: installer-shared-vpc-subnet-2
    networkProjectID: openshift-dev-installer
publish: Internal
pullSecret: <omitted>
sshKey: <omitted>