Simplify signature handling

This change brings a new SignatureHandler where Presigned.
Requests without Payload are handled very early before even
going through the call.

This change simplifies Donut codebase to not have signature related
logic for all API's.

Simplification is still needed for Payload based signature eg. PUT/POST calls
, which are still part of the donut codebase, which will be done subsequently
after donut re-write.

pkg/donut/disk/disk_linux.go

@@ -31,6 +31,7 @@ var fsType2StringMap = map[string]string{
 	"6969":     "NFS",
 	"ef51":     "EXT2OLD",
 	"ef53":     "EXT4",
+	"f15f":     "ecryptfs",
 }
 
 // getFSType - get filesystem type