add the capability for IPv6 to provider api

• in eip-service.json we need to indicate that a gateway supports ipv6

• discuss if it is sufficient to change the ip address to an ipv6 one in eip-service.json: "ip_address":"198.252.153.84"

• alternatively, we could change the ip_address field to an array, containing both ipv4 and ipv6 (I'm in favor of this option)