pluggable secret backend

Fixing secret driver serialization issue from
08f7cf05268782a0dd8e4c41a4cc65fdf78d09f2

Signed-off-by: Liron Levin <liron@twistlock.com>

api/types/swarm/secret.go

@@ -13,7 +13,7 @@ type Secret struct {
 type SecretSpec struct {
 	Annotations
 	Data   []byte  `json:",omitempty"`
-	Driver *Driver `json:"omitempty"` // name of the secrets driver used to fetch the secret's value from an external secret store
+	Driver *Driver `json:",omitempty"` // name of the secrets driver used to fetch the secret's value from an external secret store
 }
 
 // SecretReferenceFileTarget is a file target in a secret reference