Refactor mongo store handling

controller/api/initialise_test.go

controller/app/DatabaseStore.go

controller/app/Service.go

controller/app/app.go

controller/app/store.go

controller/controller.go

controller/interfaces/networkdomain/pndStore.go

controller/interfaces/networkelement/networkElementStore.go

controller/interfaces/plugin/pluginStore.go

controller/interfaces/rbac/roleStore.go

controller/interfaces/rbac/userStore.go

controller/northbound/server/topology_test.go

controller/nucleus/database/mongo-connection.go

controller/nucleus/databaseNetworkElementStore.go

controller/nucleus/databasePluginStore.go

controller/nucleus/databasePndStore.go

controller/nucleus/memoryNetworkElementStore.go

controller/nucleus/memoryPluginStore.go

controller/nucleus/memoryPndStore.go

controller/nucleus/networkElementFilesystemStore.go