fix: setup instance with human an machine user at creation (#7997)

# Which Problems Are Solved

Currently on instance setup there is only a possibility to either use a
human or a machine user and not both at creation.

# How the Problems Are Solved

The logic in the instance setup is refactored and changed so there is
not an exclusion.

# Additional Changes

Refactoring, so that unit testing is possible to add for the different
elements of an instance setup.

# Additional Context

Closes #6430

internal/command/instance_domain.go

@@ -74,7 +74,7 @@ func (c *Commands) RemoveInstanceDomain(ctx context.Context, instanceDomain stri
 }
 
 func (c *Commands) addGeneratedInstanceDomain(ctx context.Context, a *instance.Aggregate, instanceName string) ([]preparation.Validation, error) {
-	domain, err := domain.NewGeneratedInstanceDomain(instanceName, authz.GetInstance(ctx).RequestedDomain())
+	domain, err := c.GenerateDomain(instanceName, authz.GetInstance(ctx).RequestedDomain())
 	if err != nil {
 		return nil, err
 	}