kube-aws: validate user data in `kube-aws up`
kube-aws: validate user data in `kube-aws up`

fixes #382

camilb/kube-aws,everpeace/kube-aws,HotelsDotCom/kube-aws,mumoshu/kube-aws,kubernetes-incubator/kube-aws,danielfm/kube-aws,c-knowles/kube-aws,danielfm/kube-aws,mumoshu/kube-aws,AlmogBaku/kube-aws,c-knowles/kube-aws,kubernetes-incubator/kube-aws,everpeace/kube-aws,AlmogBaku/kube-aws,iflix-letsplay/kube-aws,HotelsDotCom/kube-aws,camilb/kube-aws,iflix-letsplay/kube-aws