Add basic application framework and example application to show interaction between events an NBI

Depends on:

• !340 (merged)
• !344 (merged)
• !349 (merged)
• !342 (merged)

controller/app/Service.go

+package app
+
+import (
+	"fmt"
+
+	"code.fbi.h-da.de/danet/gosdn/controller/store"
+	"github.com/google/uuid"
+)
+
+// IService is the app service.
+type IService interface {
+	Register(string, string) (*App, error)
+	Deregister(string) error
+}
+
+// Service manages apps and their access to the event system.
+type Service struct {
+	store IStore
+}
+
+// NewAppService creates a new app service.
+func NewAppService(store IStore) IService {
+	return &Service{
+		store: store,
+	}
+}
+
+// Register checks if the app already exists and if not creates a new one.
+func (a *Service) Register(appName, token string) (*App, error) {
+	if token != "SecurePresharedToken" {
+		return nil, fmt.Errorf("token not valid")
+	}
+
+	exisitingApp, err := a.store.Get(store.Query{ID: uuid.Nil, Name: appName})
+	if err != nil {
+		if exisitingApp.ID == uuid.Nil {
+			return a.createNewApp(appName)
+		}
+
+		return nil, err
+	}
+
+	return &exisitingApp, nil
+}
+
+// Deregister deregisters an app.
+func (a *Service) Deregister(appName string) error {
+	app, err := a.store.Get(store.Query{Name: appName})
+	if err != nil {
+		return err
+	}
+
+	err = a.store.Delete(app)
+	if err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func (a *Service) createNewApp(appName string) (*App, error) {
+	app := App{
+		ID:                     uuid.New(),
+		Name:                   appName,
+		EventSystemCredentials: "amqp://guest:guest@127.0.0.1:5672",
+	}
+
+	// generate app credentials
+	err := a.store.Add(app)
+	if err != nil {
+		return nil, err
+	}
+
+	return &app, nil
+}