Move ksonnet, kustomize implementations out of KfApps.Children

[kkasravi]

The kfctl Client is defined as

type KfApps struct {
	Children map[string]KfApp
}
type kfApp struct {
	kftypes.KfApps
	Client *cltypes.Client
}

where currently Ksonnet, Gcp, Minikube and other types implementing KfApp are added to
the Children member. Ksonnet is added as "none" based on legacy code in kfctl.sh.
We should redefine this to be:

type KfApps struct {
	Platforms map[string]KfApp
         PackageManagers map[string]KfApp
}
type kfApp struct {
	kftypes.KfApps
	Client *cltypes.Client
}

This would then be used to cleanly separate platforms from package managers.

[kkasravi]

/area kfctl

[kkasravi]

/priority p1

[kkasravi]

/assign @kkasravi

[kkasravi]

fixed