Reuse isReady from create_image commands

I implemented the isReady procedure for adapters for resume -- use it
in create too.

Signed-off-by: Michael Bridgen <michael@weave.works>

cmd/flux/resume.go

@@ -41,6 +41,7 @@ type resumable interface {
 	adapter
 	statusable
 	setUnsuspended()
+	successMessage() string
 }
 
 type resumeCommand struct {