should validate portgroup mode

Currently, portgroup mode accepts invalid value,
$ ironic portgroup-update a74705ad-6e77-4b42-be87-2f1d72868a25 replace mode=unknownmode
+----------------------------+--------------------------------------+
| Property | Value |
+----------------------------+--------------------------------------+
| address | 00:16:3e:79:6a:0a |
| created_at | 2017-06-06T01:15:57+00:00 |
| extra | {} |
| internal_info | {} |
| mode | unknownmode |
| name | None |
| node_uuid | bbe927a2-40ee-4546-a43c-c8469891f996 |
| properties | {} |
| standalone_ports_supported | True |
| updated_at | 2017-06-29T03:41:47+00:00 |
| uuid | a74705ad-6e77-4b42-be87-2f1d72868a25 |
+----------------------------+--------------------------------------+

In fact, ironic code should validate portgroup mode just like Linux bond driver does.