cloudapi: prevent dangling manifest goroutines

When composer exits, it doesn't wait for the manifest generation goroutines
to finish. This is generally a bad practice so let's introduce a bit of
syncing and a new Shutdown method to prevent this.

This also prevents the manifest generation goroutine from creating weird
states when interrupted on a random line of code.

Signed-off-by: Ondřej Budai <ondrej@budai.cz>

cmd/osbuild-composer/composer.go

@@ -330,7 +330,9 @@ func (c *Composer) Start() error {
 	logrus.Info("Shutting down.")
 
 	if c.apiListener != nil {
+		// First, close all listeners and then wait for all goroutines to finish.
 		err := composerAPI.Shutdown(context.Background())
+		c.api.Shutdown()
 		if err != nil {
 			panic(err)
 		}