This patch adds to legacy routers (no HA nor DVR) to be connected
to a router provider network segment through the gateway interface.
The router will be connected to one single segment of the RPN; that
means the router will have L2 connectivity to one single subnet.
The gateway router port will have an IP address on the subnet CIDR;
that will provide connectivity to the broadcast domain of this CIDR
(as usual, that doesn't change).
The router, in other scenarios, adds the other subnet CIDRs to the
router namespace routing table. That allows to SNAT any packet to
those CIDRs through the gateway port.
In the RPN case those routes are not added because there is no
broadcast connectivity with the other subnets. Any packet that needs
to reach these other subents, should go through the local segment
gateway IP address. This default route is added always into the
router namespace.
Closes-Bug: #1923592
Change-Id: Ib66b1d7b60eb0ac0a9e3dfd08aae29cb03abde34
f450886ff9