Make extended shared vpc attributes optional in project factory

[ludoo]

Using coalesce to detect whether shared_vpc_service_config is present in a project YAML definition file is problematic, since all arguments to coalesce need to have the exact same type. Which then means the variable needs to be fully fleshed out and always include the optional attributes, or the two types will not match.

Example code needed with the current module:

parent: folders/12345678
shared_vpc_service_config:
  host_project: foo-host
  service_identity_iam: {}
  service_iam_grants: []

The above is needlessly verbose, and the error message when an attribute is missing is not intuitive. The code in this PR allows specifying just the project id:

parent: folders/12345678
shared_vpc_service_config:
  host_project: foo-host