kube-aws: prefix EC2 instance names with the cluster name
kube-aws: prefix EC2 instance names with the cluster name

This change is made to ease EC2 monitoring. When we have multiple kube-aws-made clusters running on the same AWS account, if there were no prefix in their names, monitoring EC2 instances is mess because every instance is named like "kube-aws-{worker,controller}".

Fixes #371

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