Factor out export command control flow

The export command works the same way for most (all?) types. I have
made it generic and moved it into export.go, then ported
{export,create}_auto_image{repository,policy}.go to use it.

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

cmd/flux/create_auto_imagerepository.go

@@ -103,7 +103,7 @@ func createAutoImageRepositoryRun(cmd *cobra.Command, args []string) error {
 	}
 
 	if export {
-		return exportImageRepo(repo) // defined with export command
+		return printExport(exportImageRepository(&repo))
 	}
 
 	// I don't need these until attempting to upsert the object, but