Add domain and subdomain support for MinioAPI

This change brings in domain and subdomain support

   - ./minio --domain "yourminiodomain.com"

This change brings in a much needed feature by keeping
bucketnames as part of your 'DNS' name.

All your existing applications can be migrated off from s3 to
Minio without little to no modifications.

NOTE: Setting up DNS for your `buckets` is out of scope of this feature

pkg/utils/crypto/signers/signers.go

@@ -67,9 +67,9 @@ func ValidateRequest(user config.User, req *http.Request) (bool, error) {
 	encoder.Close()
 
 	// DEBUG
-	//fmt.Println("Request header sent: ", req.Header.Get("Authorization"))
-	//fmt.Println("Header calculated: ", authHeader.String())
-	//fmt.Printf("%q : %x", ss, ss)
+	// fmt.Println("Request header sent: ", req.Header.Get("Authorization"))
+	// fmt.Println("Header calculated: ", authHeader.String())
+	// fmt.Printf("%q : %x", ss, ss)
 	if req.Header.Get("Authorization") != authHeader.String() {
 		return false, fmt.Errorf("Authorization header mismatch")
 	}
@@ -155,6 +155,11 @@ var subResList = []string{"acl", "lifecycle", "location", "logging", "notificati
 // 	  <HTTP-Request-URI, from the protocol name up to the query string> +
 // 	  [ sub-resource, if present. For example "?acl", "?location", "?logging", or "?torrent"];
 func writeCanonicalizedResource(buf *bytes.Buffer, req *http.Request) {
+	bucket := bucketFromHostname(req)
+	if bucket != "" {
+		buf.WriteByte('/')
+		buf.WriteString(bucket)
+	}
 	buf.WriteString(req.URL.Path)
 	if req.URL.RawQuery != "" {
 		n := 0
@@ -176,3 +181,17 @@ func writeCanonicalizedResource(buf *bytes.Buffer, req *http.Request) {
 		}
 	}
 }
+
+func bucketFromHostname(req *http.Request) string {
+	host := req.Host
+	if host == "" {
+		host = req.URL.Host
+	}
+
+	host = strings.TrimSpace(host)
+	hostParts := strings.Split(host, ".")
+	if len(hostParts) > 1 {
+		return hostParts[0]
+	}
+	return ""
+}