log error for os.NewComputeV2

Since we are returning false, we are kind of losing the error message,
so we should at least log it.

pkg/cloudprovider/providers/openstack/openstack_instances.go

@@ -43,6 +43,7 @@ func (os *OpenStack) Instances() (cloudprovider.Instances, bool) {
 
 	compute, err := os.NewComputeV2()
 	if err != nil {
+		glog.Errorf("unable to access compute v2 API : %v", err)
 		return nil, false
 	}