Support Dual-Stack Gateway Ports on Neutron Routers

Support Dual-Stack Gateway Ports on Neutron Routers

(Patch set #2 for multiple-ipv6-prefixes blueprint)

This patchset adds support for dual-stack gateway ports on Neutron
routers. Some background on the changes included in this patchset:

- The L3 driver's init_l3() method has been changed to accept a list
  of gateway IPs, rather than a single gateway IP.
- The Neutron port dictionary's singular 'subnet' entry has been
  replaced with a 'subnets' list, since ports can now be associated
  with multiple subnets.
- The Neutron port dictionary no longer has a (singular) 'ip_cidr'
  entry, since a port can now be associated with multiple IP CIDRs
  (e.g. up to one IP CIDR per IP family on gateway ports).
  Instead, a 'prefixlen' entry has been added to the Neutron
  fixed_ips dictionary, so that the port's (multiple) IP CIDRs can
  be derived from the matching 'ip_address' and 'prefixlen' pairs
  in the port's fixed_ips.

Change-Id: I150da5938e79eeef0c947ddb1a4282e37d0515ee
Partially-implements: blueprint multiple-ipv6-prefixes