more review updates

- use /secrets for swarm secret create route
- do not specify omitempty for secret and secret reference
- simplify lookup for secret ids
- do not use pointer for secret grpc conversion

Signed-off-by: Evan Hazlett <ejhazlett@gmail.com>

errors.go

@@ -225,7 +225,7 @@ type secretNotFoundError struct {
 
 // Error returns a string representation of a secretNotFoundError
 func (e secretNotFoundError) Error() string {
-	return fmt.Sprintf("Error: No such secret: %s", e.name)
+	return fmt.Sprintf("Error: no such secret: %s", e.name)
 }
 
 // NoFound indicates that this error type is of NotFound