added a check for Public GKE clusters

[kevingair]

GKE Private clusters are almost always the right answer. NAT gateways and GLBs can be used to cover almost any use case where a cluster needs to communicate with the outside world.

[schweikert]

Thanks for your contribution! This should be probably marked as class "BP_EXT" so that it isn't enabled by default (only enabled with --include-extended).

The reason is that we don't want to recommend "blindly" to use private clusters, without knowing what it implies and how to use them. Public clusters are not deprecated or not recommended (yet, possibly).