Create the service chain with least one vm or port tuple

Currently, if any of the vm or port tuples of a service instance is not in good
state, we don't create the service chain. However, even with one of the VMs up,
the service chain can function correctly. With this chain, service chain
will only go to error if none of the VMs/port tuples are usable.

Change-Id: Ieddafb56e0b6be3a093111c10f2bdde8f486b181
Closes-Bug: 1541940

src/config/schema-transformer/config_db.py

@@ -2357,19 +2357,19 @@ def check_create(self):
                     self.log_error('virtual machine %s not found' % service_vm)
                     return None
                 vm_info = self._get_vm_pt_info(vm_obj, mode)
-                if vm_info is None:
-                    return None
-                vm_info_list.append(vm_info)
+                if vm_info:
+                    vm_info_list.append(vm_info)
             for pt in pt_list:
                 pt_obj = PortTupleST.get(pt)
                 if pt_obj is None:
                     self.log_error('virtual machine %s not found' % pt)
                     return None
                 vm_info = self._get_vm_pt_info(pt_obj, mode)
-                if vm_info is None:
-                    return None
-                vm_info_list.append(vm_info)
+                if vm_info:
+                    vm_info_list.append(vm_info)
             # end for service_vm
+            if not vm_info:
+                return None
             virtualization_type = si.get_virtualization_type()
             ret_dict[service] = {'mode': mode, 'vm_list': vm_info_list,
                                  'virtualization_type': virtualization_type}