Stop Instance creation on invalid Network spec (#466)

Before triggering the creation of a new Instance in OpenStack, the
Machine's Network section is used to fetch actual NetworkIDs from
OpenStack.

Before this change, if no OpenShift subnet was found based on the subnet
filters, the Instance spec was sent to OpenStack with an empty Network
segment, resulting in an error.

With this change, Create returns an error to the caller if no matching
networks are found in Openstack.

Author: pierreprinetti

pkg/cloud/services/compute/instance.go

@@ -130,6 +130,10 @@ func (s *Service) InstanceCreate(clusterName string, machine *clusterv1.Machine,
 			}
 		}
 	}
+	if len(nets) == 0 {
+		return nil, fmt.Errorf("no network was found or provided. Please check your machine configuration and try again")
+	}
+
 	portsList := []servers.Network{}
 	for _, net := range nets {
 		if net.networkID == "" {