ike version V2 incompatible with ike_phase1_mode

Ike version V1 divides two phases to create ike&ipsec tunnel,the ike_phase1_mode(main mode or aggresive mode) is used in first phase to negotiate ike tunnel.
Ike version v2 create ipsec sa in only one phase. If ike policy uses ike V2,it will be unnecessary to use ike_phase1_mode.
The ike policy is shown in the following,phase1_negotiation_mode should be None
root@ubuntu:~# neutron vpn-ikepolicy-show c32c991d-ecb9-460e-b829-8ce61bc8aed6
neutron CLI is deprecated and will be removed in the future. Use openstack CLI instead.
+-------------------------+--------------------------------------+
| Field | Value |
+-------------------------+--------------------------------------+
| auth_algorithm | sha1 |
| description | |
| encryption_algorithm | aes-128 |
| id | c32c991d-ecb9-460e-b829-8ce61bc8aed6 |
| ike_version | v2 |
| lifetime | {"units": "seconds", "value": 3600} |
| name | ikepolicy_a-1-1 |
| pfs | group5 |
| phase1_negotiation_mode | main |
| project_id | 899181367cc14f498f089c82c0087637 |
| tenant_id | 899181367cc14f498f089c82c0087637 |
+-------------------------+--------------------------------------+

Now the ike_phase1_mode only support main mode,it cannot be modified to set null,it need be extended to support.