S3 IAM support (#7058)

* add s3 dependencly

* rewrite image uploader by aws-sdk-go

* improve backward compatibility of s3 image uploading

* set default for expires option

* remove acl and expires

pkg/setting/setting.go

@@ -14,9 +14,10 @@ import (
 	"runtime"
 	"strings"
 
-	"github.com/go-macaron/session"
 	"gopkg.in/ini.v1"
 
+	"github.com/go-macaron/session"
+
 	"github.com/grafana/grafana/pkg/log"
 	"github.com/grafana/grafana/pkg/util"
 )