Resolve "Improve token usage for users"

Description

Related Issue

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

Resolves to #378

controller/northbound/server/auth.go

@@ -7,6 +7,7 @@ import (
 	"time"
 
 	apb "code.fbi.h-da.de/danet/gosdn/api/go/gosdn/rbac"
+	"code.fbi.h-da.de/danet/gosdn/controller/config"
 	rbacInterfaces "code.fbi.h-da.de/danet/gosdn/controller/interfaces/rbac"
 	"code.fbi.h-da.de/danet/gosdn/controller/metrics"
 	"code.fbi.h-da.de/danet/gosdn/controller/rbac"
@@ -93,7 +94,9 @@ func (s AuthServer) Login(ctx context.Context, request *apb.LoginRequest) (*apb.
 	}
 
 	userToUpdate.AddToken(token)
-
+	for len(userToUpdate.GetTokens()) > config.MaxTokensPerUser {
+		userToUpdate.RemoveToken(userToUpdate.GetTokens()[0])
+	}
 	err = s.userService.Update(userToUpdate)
 	if err != nil {
 		return nil, err