[RFE]Support VPC peering for openstack public cloud providers

Currently, the AWS VPC is a common concept for cloud providers, but for openstack models, we treat it as a Neutron Router.

So the VPC peering support corresponds to something like "Router peering" support. This function provides the network connecting between different isolated networks.

So this is a another way to connect different networks. For common use cases, we usually deploy the cloud based on overlay network, so each Router(VPC) owns its global vni(for example, we use vxlan). For vpc peering function, that means we need to support connection between different vni. So a vxlan l3 gw should be introduced into neutron for supporting network connection accross different neutron Routers. Also this is very useful for making user ease to use, for example, if a company(customer) owns mutiple accounts in openstack, and this company has multiple IT departments, each with their own isolated network, which under the neutron Router. Some networks are located within the same openstack account, and others in a different openstack account. The only way to make it happen by the Routers. You want to peer together all networks(Routers) to enable the IT departments to have full access to each others' resources.