kube-aws: fail if configPath (cluster.yaml) already exists
kube-aws: fail if configPath (cluster.yaml) already exists

If the cluster.yaml file exists and is longer than the new version being
written, the resulting cluster.yaml will contain portions of the previous
version. Open it with os.O_EXCL, causing an error if the file already
exists.

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