We should add validation in the Installer when public-only subnets is enabled to make sure that:

	1. Print a warning if OPENSHIFT_INSTALL_AWS_PUBLIC_ONLY is set
	2. If this flag is only applicable for public cluster, we could consider exit earlier if publish: Internal
	3. If this flag is only applicable for byo-vpc configuration, we could
 consider exit earlier if no subnets provided in install-config.

    all versions that support public-only subnets

    always

    1. Set OPENSHIFT_INSTALL_AWS_PUBLIC_ONLY
    2. Do a cluster install without specifying a VPC.
    3.
    

    No warning about the invalid configuration.

    This is an internal-only feature, so this validations shouldn't affect the normal path used by customers.