LGTM with one suggestion (it could use some better names though)
https://codereview.appspot.com/14307043/diff/1/provider/ec2/ec2.go
File provider/ec2/ec2.go (right):
https://codereview.appspot.com/14307043/diff/1/provider/ec2/ec2.go#newcode205
provider/ec2/ec2.go:205: if _, ok := attrs["control-bucket"]; !ok {
Thinking about mgz's remark, how about a little helper in provider/common, which
should be useful for other prepare cases too?
type DefaultAttrs map[string] func() (interface{}, error)
// AddDefaults returns the given configuration with any
// defaults in newAttr added.
func AddDefaults(cfg *config.Config, newAttr DefaultAttrs) (*config.Config,
error) {
 attrs := cfg.AllAttrs()
 for attr, makeAttr := range newAttr {
 if _, ok := attrs[attr]; ok {
 continue
 }
 new, err := makeAttr()
 if err != nil {
 return nil, err
 }
 attrs[attr] = new
 }
 return cfg.Apply(attrs)
}
Then the code here becomes:
cfg, err := common.AddDefaults(cfg, common.DefaultAttrs{
 "control-bucket": func() (interface{}, error) { return utils.NewUUID() },
})