GCP IPI supports shared VPC

1. Proposed title of this feature request
GCP IPI supports shared VPC

2. What is the nature and description of the request?
This is the same need as per: https://issues.redhat.com/browse/RFE-649 but for IPI install method.

In GCP, it is common to share VPCs in other projects in order to be used by the Openshift installer to create clusters

IPI with this topology currently is possible in UPI [1] but not in IPI method and we need to be able to support shared VPC networking.

[1] https://docs.openshift.com/container-platform/4.8/installing/installing_gcp/installing-gcp-vpc.html

3. Why does the customer need this? (List the business requirements here)
The customer made the choice to use IPI largely (datacenters and public clouds) including GCP and due to the customer internal organization, it's a requirement from network team that we use shared VPC when we create clusters on GCP.

4. List any affected packages or components.

openshift-installer on GCP