[gcp] when the optional Service Usage API is disabled, IPI installation cannot succeed

Description of problem:

According to OCP 4.11 doc (https://docs.openshift.com/container-platform/4.11/installing/installing_gcp/installing-gcp-account.html#installation-gcp-enabling-api-services_installing-gcp-account), the Service Usage API (serviceusage.googleapis.com) is an optional API service to be enabled. But, the installation cannot succeed if this API is disabled.

Applies to 4.11 and 4.12.