STS integration, JWT auth and Stateless MCS (#70)

This commit changes the authentication mechanism between mcs and minio to an sts
(security token service) schema using the user provided credentials, previously
mcs was using master credentials. With that said in order for you to
login to MCS as an admin your user must exists first on minio and have enough
privileges to do administrative operations.

```
./mc admin user add myminio alevsk alevsk12345
```

```
cat admin.json

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "admin:*",
        "s3:*"
      ],
      "Resource": [
        "arn:aws:s3:::*"
      ]
    }
  ]
}

./mc admin policy add myminio admin admin.json
```

```
./mc admin policy set myminio admin user=alevsk
```

restapi/user_logout_test.go

@@ -1,21 +1,29 @@
+// This file is part of MinIO Console Server
+// Copyright (c) 2020 MinIO, Inc.
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
 package restapi
 
-import (
-	"testing"
+import "testing"
 
-	mcCmd "github.com/minio/mc/cmd"
-	"github.com/minio/mcs/restapi/sessions"
-)
-
-// TestLogout tests the case of deleting a valid session id
-func TestLogout(t *testing.T) {
-	cfg := mcCmd.Config{}
-	// Creating a new session
-	sessionID := sessions.GetInstance().NewSession(&cfg)
-	// Test Case 1: Delete a session Valid sessionID
-	function := "logout()"
-	err := logout(sessionID)
-	if err != nil {
-		t.Errorf("Failed on %s:, error occurred: %s", function, err.Error())
-	}
+// mock function of Get()
+func (ac mcsCredentialsMock) Expire() {
+	// Do nothing
+	// Implementing this method for the mcsCredentials interface
+}
+
+func TestLogout(t *testing.T) {
+	// There's nothing to test right now
 }