Dualstack support for L3VPN-two router design

Dualstack support for L3VPN-two router design

According to dualstack specification [0], this commit introduces
following changes:

* neutronvpn.yang: Use "router-ids" leaf-list instead of "router-ids"
  leaf in grouping "vpn-instance" and "vpnMaps" container. Two neutron
  routers can be attached to one L3 BGPVPN instance at the same time:
  the first one with only IPv4 ports and the second one with only IPv6
  ports.

* NeutronvpnManager, NeutronvpnUtils: Support routerIds list in all
  basic methods. Control the number of associated routers in "add" and
  "handleRoutersUpdate" methods. Consequently assign each router
  accordingly.

* IVpnLinkServiceImpl: Appropriate changes, due to support of routerIds
  list in neutronvpn.yang.

* NatUtil:

  * "getRouterIdfromVpnInstance": control, that we get routerId from
    appropriate IPv4 Subnetmap, which is corresponding to given vpnId;
  * "getVpnForRouter" support routerIdsList, due to  changes in
    neutronvpn.yang.

* neutronvpn: rpc L3VPN support for assoc/disassoc routers list

Change yang model to associate or dissociate two routers with a vpnid in
the same curl query.

Change-Id: Iade14e52baee2da4a82f01231c7ea6f8051e520b
Signed-off-by: Noel de Prandières <prandieres@6wind.com>
Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>
Signed-off-by: Karthikeyan Krishnan <karthikeyangceb007@gmail.com>