Add support for configs

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

cli/command/service/parse.go

@@ -57,3 +57,53 @@ func ParseSecrets(client client.SecretAPIClient, requestedSecrets []*swarmtypes.
 
 	return addedSecrets, nil
 }
+
+// ParseConfigs retrieves the configs from the requested names and converts
+// them to config references to use with the spec
+func ParseConfigs(client client.ConfigAPIClient, requestedConfigs []*swarmtypes.ConfigReference) ([]*swarmtypes.ConfigReference, error) {
+	configRefs := make(map[string]*swarmtypes.ConfigReference)
+	ctx := context.Background()
+
+	for _, config := range requestedConfigs {
+		if _, exists := configRefs[config.File.Name]; exists {
+			return nil, errors.Errorf("duplicate config target for %s not allowed", config.ConfigName)
+		}
+
+		configRef := new(swarmtypes.ConfigReference)
+		*configRef = *config
+		configRefs[config.File.Name] = configRef
+	}
+
+	args := filters.NewArgs()
+	for _, s := range configRefs {
+		args.Add("name", s.ConfigName)
+	}
+
+	configs, err := client.ConfigList(ctx, types.ConfigListOptions{
+		Filters: args,
+	})
+	if err != nil {
+		return nil, err
+	}
+
+	foundConfigs := make(map[string]string)
+	for _, config := range configs {
+		foundConfigs[config.Spec.Annotations.Name] = config.ID
+	}
+
+	addedConfigs := []*swarmtypes.ConfigReference{}
+
+	for _, ref := range configRefs {
+		id, ok := foundConfigs[ref.ConfigName]
+		if !ok {
+			return nil, errors.Errorf("config not found: %s", ref.ConfigName)
+		}
+
+		// set the id for the ref to properly assign in swarm
+		// since swarm needs the ID instead of the name
+		ref.ConfigID = id
+		addedConfigs = append(addedConfigs, ref)
+	}
+
+	return addedConfigs, nil
+}