Subnet create and subnet show list additional IPV6 fields even if IP_Version is 4

[Vijendra Soni]

Additional fields(ipv6_ra_mode, ipv6_address_mode) in Subnet create and Subnet show even if ip_version=4

openstack subnet show Subnet-RED
+-------------------+--------------------------------------+
| Field | Value |
+-------------------+--------------------------------------+
| allocation_pools | 10.0.1.2-10.0.1.254 |
| cidr | 10.0.1.0/24 |
| created_at | 2016-12-03T13:07:16Z |
| description | |
| dns_nameservers | |
| enable_dhcp | True |
| gateway_ip | 10.0.1.1 |
| host_routes | |
| id | 2e96c40f-3c16-403c-a120-11e4328c8c9e |
| ip_version | 4 |
| ipv6_address_mode | None |
| ipv6_ra_mode | None |
| name | Subnet-RED |
| network_id | 00ca0d7b-6bf7-40b5-9985-6d4cb2211495 |
| project_id | 734d93dc373e4b6fad6effaf33db689b |
| project_id | 734d93dc373e4b6fad6effaf33db689b |
| revision_number | 2 |
| service_types | |
| subnetpool_id | 3ba5e12c-5dfa-4776-bd3d-8ac19a96172c |
| updated_at | 2016-12-03T13:07:16Z |
+-------------------+--------------------------------------+

We should not display IPV6 fields((ipv6_ra_mode, ipv6_address_mode) if ip_version=4