kube-aws: Print clearer message on `kube-aws destroy`
kube-aws: Print clearer message on `kube-aws destroy`

Without this change, `kube-aws destroy` immediately returns with the message denoting that the cfn stack is already destroyed (which is not true as it takes several minutes until the stack get actually destroyed).

See https://github.com/coreos/coreos-kubernetes/issues/434#issuecomment-215514237 for discussion regarding phrasing and words.

Fixes #434

amarruedo/coreos-kubernetes,chen-anders/coreos-kubernetes,amarruedo/coreos-kubernetes,ah45/coreos-kubernetes,danielfm/coreos-kubernetes,amarruedo/coreos-kubernetes,ah45/coreos-kubernetes,ah45/coreos-kubernetes,chen-anders/coreos-kubernetes,limres/coreos-kubernetes,bnea-engineering/coreos-kubernetes,danielfm/coreos-kubernetes,limres/coreos-kubernetes,chen-anders/coreos-kubernetes,bnea-engineering/coreos-kubernetes,coreos/coreos-kubernetes,bnea-engineering/coreos-kubernetes,joshix/coreos-kubernetes,amarruedo/coreos-kubernetes,limres/coreos-kubernetes,IDKA/coreos-kubernetes,IDKA/coreos-kubernetes,turbonomic/coreos-kubernetes,coreos/coreos-kubernetes,IDKA/coreos-kubernetes,turbonomic/coreos-kubernetes,danielfm/coreos-kubernetes,turbonomic/coreos-kubernetes,joshix/coreos-kubernetes