add support for build tag to allow to disable some features

The following build tags are available:

- "nogcs", disable Google Cloud Storage backend
- "nos3", disable S3 Compabible Object Storage backends
- "nobolt", disable Bolt data provider
- "nomysql", disable MySQL data provider
- "nopgsql", disable PostgreSQL data provider
- "nosqlite", disable SQLite data provider
- "noportable", disable portable mode

dataprovider/bolt.go

@@ -1,3 +1,5 @@
+// +build !nobolt
+
 package dataprovider
 
 import (
@@ -52,6 +54,10 @@ type compatUserV2 struct {
 	Status            int      `json:"status"`
 }
 
+func init() {
+	utils.AddFeature("+bolt")
+}
+
 func initializeBoltProvider(basePath string) error {
 	var err error
 	logSender = fmt.Sprintf("dataprovider_%v", BoltDataProviderName)