Neutron: Fix IPv6 provider networks
Neutron: Fix IPv6 provider networks

The creation of the IPv6 subnet with provider networks does not
specifiy the correct parameters:
 . V6_NETWORK_GATEWAY is specified but never set
       - created new IPV6_PROVIDER_NETWORK_GATEWAY
 . PROVIDER_SUBNET_NAME_V6 is specified but never set
       - created new IPV6_PROVIDER_SUBNET_NAME
 . FIXED_RANGE_V6 is being used
       - created new IPV6_PROVIDER_FIXED_RANGE
 . subnetpool_id is incorrect
       - changed to subnetpool

Related-Bug: #1507870
Closes-Bug: #1417410
Change-Id: I9a1ad11bc54529080ec84d4677fa5633708890c7

bigswitch/devstack,dtroyer/devstack,dirkmueller/devstack,noironetworks/devstack,dirkmueller/devstack,Mahito/devstack,dtroyer/devstack,nawawi/openstack,openstack-dev/devstack,avvocatodemarchis/devstack,avvocatodemarchis/devstack,samgoon/devstack,bigswitch/devstack,richliu/devstack,nawawi/openstack,bswartz/devstack,noironetworks/devstack,samgoon/devstack,mssumanth/devstack,richliu/devstack,group-policy/devstack,mssumanth/devstack,openstack-dev/devstack,bswartz/devstack,Mahito/devstack,group-policy/devstack