In Neutron VPNaaS IKE and IPSEC policies the lists of choices for encryption algorithms, auth algorithms and PFS modes was extended in neutron-lib v3.9.0 and v3.18.0.
Add the support for the new ciphers in gophercloud.

Encryption algorithms: Several variants for AES CCM and AES GCM modes were added (see https://opendev.org/openstack/neutron-lib/src/commit/bf21a6dcd48bdd15c28086f256319ac035b7fef0/neutron_lib/api/definitions/vpn.py#L47), AES CTR modes (see https://opendev.org/openstack/neutron-lib/src/branch/master/neutron_lib/api/definitions/vpn_aes_ctr.py)

Auth algorithms: aes-xcbc abd aes-cmac (see https://opendev.org/openstack/neutron-lib/src/commit/bf21a6dcd48bdd15c28086f256319ac035b7fef0/neutron_lib/api/definitions/vpn.py#L178)

PFS: Diffie Hellman groups 15 to 31 (see https://opendev.org/openstack/neutron-lib/src/branch/master/neutron_lib/api/definitions/vpn_aes_ctr.py)